Opinion for the Court filed by Circuit Judge SENTELLE.
Opinion dissenting in part filed by Circuit Judge GARLAND.
SENTELLE, Circuit Judge:
Appellant Richard Spinner challenges his conviction on four weapons and narcotics charges. We agree with Spinner that the government introduced insufficient evidence to sustain his conviction for possession of a semiautomatic assault weapon, and accordingly reverse that conviction. And because the district court permitted the prosecutor to ask a defense witness a series of inappropriate questions on' cross-examination, we reverse and remand Spinner’s conviction for possession with intent to distribute crack cocaine within 1,000 feet of a school. We affirm the remaining convictions.
L. Background
A. The Offense
On August 8, 1996, at approximately 4:00 p.m., Washington Metropolitan Police Department officers and FBI agents executed a search warrant at 636 46th Place, S.E., in the District of Columbia. Four people were present at the time: Richard Spinner, his mother, his 16-year-old sister, and his 17-year-old cousin.
The officers discovered three loaded guns during the course of their search. Two of them — a .380 caliber Colt semiautomatic pistol and a .45 caliber Sturm-Ruger semiautomatic .pistol — were found under the cushions of a couch in the living room. The third gun, a Colt .223. caliber semiautomatic rifle, was found in the closet of a second-floor bedroom, inside a rifle case. During their search of that closet, the officers also recovered sever[953]*953al .223 caliber magazines of ammunition, a bulletproof vest, and two ski masks. In addition, the officers found 1.279 grams of crack cocaine, packaged in multiple ziplock bags, in the living room and second-floor bedroom.
The officers recovered documents relating to Spinner from the upstairs bedroom, including correspondence addressed to him; receipts bearing his name; his social security card; and a list — handwritten on an envelope bearing Spinner’s fingerprint — of current prices for various quantities of crack cocaine. Spinner’s fingerprints were also found on two other noteworthy items: a .45 caliber bullet, which was inside the .45 caliber pistol; and a box of .44 caliber bullets found in the closet where the semiautomatic rifle was recovered. The officers also found two photographs that depicted Spinner in the upstairs bedroom.
As a result of the officers’ search, a federal grand jury returned a five-count criminal indictment against Spinner. The indictment charged Spinner with two counts of violating 18 U.S.C. § 922(g)(1), which makes it illegal for a convicted felon to possess “any firearm or ammunition.” (Spinner had a 1993 felony conviction for possession of a firearm with a removed, obliterated, or altered serial number in violation of 18 U.S.C. § 922(k).) The indictment also charged Spinner with possession of a semiautomatic assault weapon, 18 U.S.C. § 922(v)(l); possession with intent to distribute cocaine base within 1,000 feet of a school, 21 U.S.C. § 860(a); and possession with intent to distribute cocaine base, 21 U.S.C. §§ 841(a)(1) and 841(b)(1)(C). No charges were brought against Spinner’s sister or his mother. However, Spinner’s cousin was prosecuted in a separate proceeding in juvenile court.
B. The Trial
The government sought to prove that Spinner constructively possessed the recovered contraband. It argued that Spinner had access to the upstairs bedroom, and the ability to control the contraband that was found there. In support of this position, it introduced into evidence Spinner’s personal papers that were found in the bedroom, as well as the photographs depicting Spinner in the bedroom. To prove that Spinner possessed the contraband intentionally, the government stressed the presence of Spinner’s fingerprints on the .45 caliber bullet, the box of .44 caliber ammunition, and the drug price list. It also introduced evidence of Spinner’s “other crimes” pursuant to Federal Rule of Evidence 404(b) in order to show Spinner’s intent to possess the contraband. In particular, the jury heard evidence that prior to his arrest Spinner had unlawfully possessed a semiautomatic handgun with an obliterated serial number, and that he had sold 25 zi-plock bags of crack to an undercover officer in front of the house at which the search warrant was executed.
To make the case that the weapon recovered from the closet of the upstairs bedroom met the statutory definition of “semiautomatic assault weapon,” the government introduced the testimony of Richard A. Turner, a firearms enforcement officer employed by the Bureau of Alcohol, Tobacco and Firearms. The district court permitted Turner to testify as an expert “concerning firearms, ammunition, identification, operation and design.”
The statutory term “semiautomatic assault weapon” includes:
a semiautomatic rifle that has an ability to accept a detachable magazine and has at least 2 of—
(i) a folding or telescoping stock;
(ii) a pistol grip that protrudes conspicuously beneath the action of the weapon;
(iii) a bayonet mount;
(iv) a flash suppressor or threaded barrel designed to accommodate a flash suppressor; and
(v) a grenade launcher.
18 U.S.C. § 921(a)(30)(B).
In response to the prosecutor’s questions, Turner described the recovered weapon, which is commonly called an AR-15 rifle, as a “semiautomatic rifle which can accept a detachable magazine.... ” This testimony more or less tracks the statutory phrase: “a semiautomatic rifle that has- an ability to accept a detachable magazine.” Next, the prosecutor asked Turner: “And what are the features, just in general, that would turn this [954]*954[particular] weapon into a semiautomatic assault weapon?” Turner responded:
Well, it has a telescoping shoulder stock. So that’s one feature. And then it has a pistol grip that extends beyond the bottom of the receiver.... By having these two features, it would put it into the classification of a semiautomatic assault weapon.
(emphasis added). Here, Turner’s language diverged from the language of the statute, which refers to a “pistol grip that protrudes conspiauously beneath the action of the weapon.” The prosecutor did not ask Turner to explain what he meant by “receiver,” nor did she ask him whether that term was equivalent to the statutory term “action.” Nor indeed did she ask any follow-up questions about Turner’s conclusion. And while the record makes it clear that Turner had the weapon at issue in front of him on the witness stand, it is not apparent whether he pointed to the “bottom of the receiver” as he spoke.
Spinner’s theory of the case was that he had not lived at the house for several months prior to his arrest, and that his cousin solely possessed the contraband. Both Spinner’s cousin and his sister testified that the cousin had stayed in the upstairs bedroom frequently during the summer when Spinner was arrested. ' The cousin testified that he had hidden the guns, drugs and ammunition in the house himself, hoping perhaps to sell them in the future.
To establish that Spinner was not living at the house when the arrest took place, the defense called to the stand Spinner’s girlfriend, Lolita Little. On direct examination, Ms. Little testified that Spinner had moved in with her in late June 1996, approximately two months before Spinner’s arrest. On cross-examination, the prosecutor asked her if “last May” she had been “upset that Richard Spinner was letting his fiiend use his mother’s house to sell drugs.” Defense counsel objected. At the bench, the prosecutor explained that her “good faith basis” for asking the question was a letter dated May 13,1996, that Ms. Little had written Spinner when he was incarcerated for an unrelated crime. In pertinent part, the letter, with expletives deleted, said:
I hope your thoughts be straight and stop those m* * * * *f* * * * *s from using your mother and her home to do this s* * * because you wouldn’t do that in their mother’s s* * *, they wouldn’t do that s* * * to their mothers out of respect.
The prosecutor explained that she intended to rebut the defense’s suggestion in its opening statement that Spinner had “changed his life around” prior to “last May” by showing that Spinner had let his friend use his mother’s house to sell drugs. She added that the defense is “asking the jury to believe [Spinner] changed his life and I think it’s — the government should have the opportunity, first of all, for the jury to know what he was doing before this.” She also argued that the challenged question was “entirely relevant to the government’s argument that Mr. Spinner knew that [his cousin] was using, was selling drugs out of this premises.” The defense objected to the question on the grounds that it was prejudicial, that it exceeded the scope of direct examination, and that it arguably constituted Rule 404(b) evidence for which the government did not provide sufficient notice.
The trial judge- said he would permit the prosecutor to pursue this line of questioning because it undermined the credibility of the witness; indeed, he even urged the government to prosecute Ms. Little for perjury because he believed that the letter contradicted her earlier testimony. The judge also said that he would permit the prosecutor to proceed because the letter was evidence of Spinner’s “other crimes, wrongs, or acts” under Rule 404(b) of the Federal Rules of Evidence because it related to his intent to distribute the narcotics seized from the house. The trial judge also found good cause to excuse Rule 404(b)’s requirement that the government provide reasonable notice of its intent to use the evidence, remarking that “when something arises in the defense case that was unexpected, that’s a basis to excuse pretrial notice.”
As the district judge was announcing this ruling, the lawyers realized that the judge mistakenly believed that Ms. Little had testified that she had never seen drugs at the [955]*955house or seen Spinner with drugs. The judge, it turns out, had confused Ms. Little with another witness who had so testified. When the lawyers pointed out the mistake, the trial judge nevertheless permitted the prosecutor to use the letter as a basis for cross-examination. (The letter itself was never admitted into evidence, and is not part of the record before us.)
Responding to the prosecutor’s questions, Ms. Little denied seeing drugs around the house, and denied that her letter to Spinner referred to drugs. She interpreted her letter to mean that she wished Spinner would stop people from “drinking and' cursing and laying up in his mother[’s] house.” The prosecutor also paraphrased a passage from the letter stating that Spinner “was no better than the friends that he let take advantage of his mother because he was greedy.” Ms. Little denied that this passage referred to drugs. Finally, the prosecutor referred Ms. Little to a particular sentence of the letter, and asked: “Now, you were concerned in that sentence that if Mr. Spinner continued to do the s* * * that you referred to, he would end up locked up or dead, isn’t that right?” Ms. Little assented, and reiterated that the reference to “s* * * ” meant “cursing and drinking and laying around the house.”
The jury found Spinner guilty on all of the counts of the indictment. At the sentencing hearing, the district court dismissed Count Five (possession with intent to distribute cocaine) because it was a lesser included offense of Count Four (possession with intent to distribute cocaine within 1,000 feet of a school). The district court sentenced Spinner to 92 months of imprisonment on Counts One and Two (the felon-in-possession counts) and Four, to be served concurrently. The court also imposed a 60-month concurrent sentence for Count Three, which was for possession of a semiautomatic assault weapon.
Spinner filed a timely notice of appeal.
II. Discussion
A. Sufficiency of the Evidence
Spinner argues that the evidence was not sufficient to support his conviction for possession of a semiautomatic assault weapon under 18 U.S.C. § 922(v). He avers that the government failed to prove that the AR-15 meets the statutory requirements for being a prohibited weapon. Citing Staples v. United States, 511 U.S. 600, 114 S.Ct. 1793, 128 L.Ed.2d 608 (1994), he also argues that the government failed to meet its burden of proving beyond a reasonable doubt that he possessed the required mens rea for conviction. Staples clarified the government’s obligation to prove that a defendant knew that a given firearm had the characteristics that brought it within the scope of the statute rendering possession of the weapon unlawful.
1. Standard of Review
Our standard of review depends on whether Spinner preserved these arguments by moving for judgment of acquittal pursuant to Federal Rule of Criminal Procedure 29. Our precedents recognize that a “broadly stated” motion for judgment of acquittal “without specific grounds” is “sufficient to preserve [a] full range of challenges ... to the sufficiency of the evidence.” United States v. Hammoude, 51 F.3d 288, 291 (D.C.Cir.1995); see also United States v. Milton, 8 F.3d 39, 45 (D.C.Cir.1993) (a “general claim of insufficient evidence” was sufficient to preserve a specific point of error not raised below). However, we review an appellant’s suffieiency-of-the-evidence challenge for plain error when a motion for judgment of acquittal was based on specific (and different) grounds. United States v. Sayan, 968 F.2d 55, 62 (D.C.Cir.1992); see also United States v. White, 1 F.3d 13, 17 (D.C.Cir.1993) (recognizing that sufficiency-of-the-evidenee challenge would be reviewed for plain error if no motion for judgment of acquittal was filed with respect to the counts of appellant’s con.vietion); but see United States v. Gjurashaj, 706 F.2d 395, 399 (2nd Cir.1983) (“[W]hen a defendant moves for acquittal, even without specificity as to the grounds, it is incumbent upon the government to review its proof as to the facts required, to establish each element of each offense alleged.”).
Spinner moved for judgment of acquittal at trial. Addressing the semiautomatic assault weapon charge, his lawyer complained that “there has been no evidence [956]*956submitted whatsoever to indicate any posses-sory interest of my client in that firearm.” He explained to the district court that “there has been no testimony of [Spinner’s] fingerprints [on the AR-15], any testimony of [Spinner’s] prior contact [with the AR-15].” He did not, however, suggest that the government failed to shoulder its obligations under Staples, nor did he intimate that the government had fallen short of proving that the recovered weapon met the statutory test for unlawfulness. Because Spinner did not raise before the district court the specific arguments he raises before us, we review them for plain error under Sayan.
Under Rule 52(b) of the Federal Rules of Criminal Procedure, “[p]lain errors or defects affecting substantial rights may be noticed although they were not brought to the attention of the court.” To be “noticed” under this rule, an error must be “plain” (or, in other words, “obvious”) and “must have affected the outcome of the District Court proceedings.” United States v. Clarke, 24 F.3d 257, 266 (D.C.Cir.1994) (quoting United States v. Olano, 507 U.S. 725, 734, 113 S.Ct. 1770, 123 L.Ed.2d 508 (1993)). When reviewing a sufficiency-of-the-evidence challenge for plain error; we- reverse only to prevent a “manifest miscarriage of justice.” United States v. Jackson, 824 F.2d 21, 26 (D.C.Cir.1987) (quoting United States v. Baber, 447 F.2d 1267, 1270 n. 8 (D.C.Cir.1971)). Such a miscarriage would exist “only if the record is devoid of evidence pointing to guilt, or ... because the evidence on a key element of the offense was so tenuous that a conviction would be shocking.” United States v. Parker, 133 F.3d 322, 328 (5th Cir.1998) (citation omitted); accord United States v. Wright, 63 F.3d 1067, 1074 (11th Cir.1995); United States v. Meadows, 91 F.3d 851, 855 (7th Cir.1996). It would be a manifest miscarriage of justice to let a conviction stand if the government failed to present any evidence on an essential element of the crime. Id.; see also Beckett v. United States, 379 F.2d 863, 864 (9th Cir.1967) (finding a manifest miscarriage of justice where “there wás no proof of one of the essential elements” of the crimes charged).
In White, we expressed uncertainty as to how a plain error review of a suffieiency-of-the-evidenee argument might differ from the standard of review we apply when the argument has been preserved. 1 F.3d at 17. When applying the latter standard, we determine, after viewing the evidence in the light most favorable to the prosecution, whether “any rational trier of fact could have found the essential elements of the crime beyond a reasonable doubt.” Id. (citing Jackson v. Virginia, 443 U.S. 307, 319, 99 S.Ct. 2781, 61 L.Ed.2d 560 (1979)). That standard is itself “highly deferential.” United States v. Lucas, 67 F.3d 956, 959 (D.C.Cir.1995). In White, we expressed difficulty in imagining a standard of review any more deferential than the Jackson standard. See 1 F.3d at 17; see also United States v. Pennington, 20 F.3d 593, 597 n. 2 (5th Cir.1994).
In any event, we need not resolve whether, or in what respect, the plain error standard might differ from the Jackson standard in the context of a challenge to the sufficiency of the evidence supporting a conviction, because we hold that the government failed to present any evidence on an essential element of a crime for which Spinner was convicted. Such a lapse would warrant reversal under either standard. See Meadows, 91 F.3d at 855 n. 6 (“[A] complete lack of any evidence of one of the essential elements of a crime is not only insufficient evidence, but too little evidence to avoid a manifest miscarriage of justice.”). .
2. Did the Government Prove that the AR-15 was a Prohibited Weapon?
First, we address the government’s efforts to prove that the recovered AR-15 met the statutory definition of a prohibited semiautomatic assault weapon.
It is axiomatic that the government bears the burden of proving all elements of a crime beyond a reasonable doubt. Indeed, the Due Process Clause of the Fifth Amendment “protects the accused against conviction except upon proof beyond a reasonable doubt of every fact necessary to constitute the crime with which he is charged.” In re Winship, 397 U.S. 358, 364, 90 S.Ct. 1068, 25 L.Ed.2d 368 (1970). Here, Spinner was convicted of violating 18 U.S.C. § 922(v)(l), which criminalizes the possession of a “semiautomatic assault weapon.” A semiautomatic rifle, like that recovered from the upstairs [957]*957bedroom in this case, is such an unlawful weapon if it is able to accept a detachable magazine and has at least two of the following features:
(i) a folding or telescoping stock;
(ii) a pistol grip that protrudes conspicuously beneath the action of the weapon;
(iii) a bayonet mount;
(iv) a flash suppressor or threaded barrel designed to accommodate a flash suppressor; and
(v) a grenade launcher.
18 U.S.C. § 921(a)(30)(B). To obtain a conviction under section 922(v)(l), the government must prove beyond a reasonable doubt that the recovered weapon satisfied the statutory requirements. In addition, as we shall discuss below, it must also prove beyond a reasonable doubt that Spinner knew that the recovered firearm possessed the characteristics that brought it within the scope of the statute. See Staples v. United States, 511 U.S. 600, 619, 114 S.Ct. 1793, 128 L.Ed.2d 608 (1994).
The government sought to prove that the AR-15 possessed two (and only two) of the five enumerated statutory features, namely a “folding or telescoping stock,” and a “pistol grip that protrudes conspicuously beneath the action of the weapon.” It is undisputed that the government proved that the AR-15 was able to accept a detachable magazine, and that it possessed a “folding or telescoping stock.” Therefore, the only issue itere is whether the government has proved that the AR-15 possessed a “pistol grip that protrudes conspicuously beneath the action of the weapon.”1
The government’s expert on firearms testified that the AR-15 had a “pistol grip that extends beyond the bottom of the receiver.” On appeal, the government has furnished us with a glossary published by the Associations of Firearms and Toolmark Examiners, which defines “receiver” as “[t]he basic unit of a firearm which houses the firing and breech mechanism and to which the barrel and stock are assembled.” It is telling, we think, that the government has submitted a technical publication to explain the meaning of the term “receiver.” Turner clearly used the word as a term of art, and not in a commonly understood sense. Since the prosecutor never asked Turner to explain what he meant by “receiver,” and never asked Turner whether “receiver” was equivalent to the statutory term “action,” the jury was not given any evidence to support a conclusion that the AR-15 possessed “a pistol grip that protrudes conspicuously beneath the action of the weapon.” 18 U.S.C. § 921(a)(30)(B)(ii). This evidentiary vacuum on a key statutory element requires that we reverse the conviction.
The government argues that the jury had the opportunity to see for itself whether the AR-15 possessed the required statutory features. The jury, this argument goes, could have examined the gun during its deliberations, and seen that the pistol grip [958]*958protruded “conspicuously beneath the action of the weapon.” If the jury could have puzzled this out on its own, why would it matter that the testimony of the expert was deficient? In Meadows, the Seventh Circuit confronted a strikingly similar argument, and rejected it. 91 F.3d at 856. The weapon at issue in that case was a World War I era pistol or revolver that the defendant had modified by adding a stock that apparently enabled the weapon to be fired from the shoulder. Based on the testimony of an ATF expert that the weapon was unlawful, the defendant was convicted of possessing a firearm that was made in violation of the provisions of 26 U.S.C. § 5861(c). That statute defines the weapons it prohibits as follows:
The term “rifle” means a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fixed cartridge to fire only a single projectile through a rifled bore for each single pull of the trigger, and shall include any such weapon which may be readily restored to fire a fixed cartridge.
26 U.S.C. § 5845(c) (emphasis added).
At trial, the ATF expert did not define or explain the term “rifled bore.” Meadows, 91 F.3d at 853. Referring to a dictionary definition, the Seventh Circuit explained that “the term ‘rifled’ is derived from the verb ‘to rifle,’ which means ‘to cut spiral grooves into the bore of (as a firearm- or piece of ordnance).” Id. at 856 (quoting Webster’s Third New International Dictionary 1954 (1986)). The Meadows court concluded that the absence of any testimony that the gun in question possessed a “rifled bore” — as well as the absence of any explanation as to what that term meant — was a fatal flaw in the government’s case. Noting that the modified weapon at issue could well have had a smooth or a rifled bore, the court observed that “[tjhere was no indication in the testimony that the bore was rifled, or that rifling was present in the bore.” Id. at 857.
On appeal, the government argued that, notwithstanding the lack of testimony concerning the rifled bore, the jury simply could have looked down the barrel of the weapon during its deliberations, and determined on its own whether the bore was smooth or rifled. This argument failed to persuade the Seventh Circuit. “[W]hy,” asked the court, “would the jurors have bothered to look down the barrel to determine if the bore was ‘rifled,’ if they had no explanation of what ‘rifled’ meant?” Id. It continued: “We do not see why a jury would look for a feature of the weapon that neither the parties, the witnesses, nor the judge suggested that the jury should examine.” Id. Accordingly, the Meadowseourt concluded that there was “a complete gap in the evidence regarding this element.” Id.
Since the defendant in Meadowshad not preserved this objection by. filing a proper motion for judgment of acquittal, the court applied a plain error standard of review. See id. at 854-55. Stressing the government’s failure to prove that the weapon had a rifled bore and its additional failure to prove that the defendant knew that the weapon had this feature that made possession unlawful, see Staples, supra, the court reversed, concluding that allowing the defendant’s conviction to stand would amount to a miscarriage of justice. Id. at 857.
Like the Seventh Circuit in Meadows, we also reject the government’s argument that the jury, without any testimonial guidance, could have determined that the weapon satisfied the applicable statutory requirements. Granted, the jury in this case had the opportunity to examine the AR-15 during its deliberations. But without any explanation of the meaning of the statutory term “action,” the jury had no evidentiary basis to conclude that the AR-15 met the statutory definition of a prohibited semiautomatic assault weapon. Indeed, not only was the term “action” never explained to the jury, the record reflects that the jury heard this term for the first time when the district court delivered its instructions.2
[959]*9593. Did the Government Prove the Required Mens Rea for Conviction?
Spinner argues that even if the government had proved that the AR-15 was a prohibited semiautomatic weapon, it failed to prove that he possessed the required mens rea of section 922(v). In Staples v. United States, the Supreme Court held that under 26 U.S.C. § 5861(d), which prohibits the possession of an unregistered automatic firearm, the government must prove beyond a reasonable doubt that a defendant knew of the features of the weapon that brought it within the scope of the statute. 511 U.S. 600, 619, 114 S.Ct. 1793, 128 L.Ed.2d 608 (1994). The government concedes that the rationale of Staples applies to section 922(v) as well. Thus, the government must prove that Spinner knew of the features of the AR-15 that brought it within the scope of section 922(v).
Given our conclusion that the government failed to prove that the recovered weapon met the statutory definition of a forbidden semiautomatic assault weapon, it follows that the government similarly failed to prove beyond a reasonable doubt that Spinner knew that the AR-15 possessed the required statutory characteristics. Furthermore, although such knowledge “can be inferred from circumstantial evidence, including any external indications signaling the nature of the weapon,” Staples, 511 U.S. at 615-16 n. 11, 114 S.Ct. 1793, we think the circumstantial evidence that Spinner knew of the AR-15’s statutory characteristics is very thin. When he was arrested, Spinner was in the kitchen. The AR-15 was found in a rifle ease in a closet in the upstairs bedroom. The authorities discovered no fingerprints linking Spinner to the weapon or the ammunition inside it. Although the bedroom contained numerous documents bearing Spinner’s name (all but one of them dated prior to the month of the arrest), it also contained documents bearing the names of three other family members. The government argues that “[t]he jury could have inferred that [Spinner] had an unusually strong interest in assault weapons based on the discovery in the [upstairs bedroom] of a magazine advertisement for an entire book about an assault weapon.” But there was no evidence presented that Spinner had even seen this article. Finally, both Spinner’s cousin and his girlfriend testified that Spinner had moved to his girlfriend’s house in June, 1996, two months before Spinner’s arrest. Clearly, this evidentiary record falls far short of that in United States v. Moore, in which we found that evidence establishing that the defendant had handled a rifle and “was in continuous control of the weapon” was sufficient for a jury to conclude that the defendant had the requisite mens rea for possession of a sawed-off shotgun under 26 U.S.C. § 5861(d). 97 F.3d 561, 563-64 (D.C.Cir.1996).
Given the complete lack of evidence in the record that the AR-15 possessed “a pistol grip that protrudes conspicuously beneath the action of the weapon,” 18 U.S.C. § 921(a)(30)(B)(ii), as well as the paucity of evidence that Spinner knew that the weapon possessed the features that brought it within the scope of the statute, we conclude that Spinner’s conviction on this charge was a manifest miscarriage of justice, and we reverse it.
Because we reverse Spinner’s conviction as a result of the insufficiency of the evidence supporting it, we need not pass on Spinner’s argument that the jury instructions failed to explain the government’s obligation to prove Spinner’s mens rea under Staples. Nonetheless, in order to alert the district courts to this issue, we reproduce the jury instructions below:
Count three of the indictment charges unlawful possession of a semiautomatic assault weapon. The essential elements of this offense ... are, one, that the defendant possessed a semiautomatic assault weapon and, two, that he did so knowingly and intentionally. A person, as I have said, acts knowingly and intentionally if he’s conscious and aware of his act, realizes what he was doing and does not act because of mistake, inadvertence or accident. The term semiautomatic [960]*960assault weapon means a semiautomatic rifle that has an ability to accept a detachable magazine and has at least two of the following: [elements omitted]. It has to have at least two of those.
As the government acknowledges, “at worst, the[se] jury instructions may have insufficiently explained the scienter requirement regarding the relevant features of the rifle.” Without taking any position on whether these instructions would have survived our review of them for plain error, we encourage the district courts to explicitly instruct juries, when appropriate, that the government must prove that a defendant knew of the particular features of a weapon that rendered its possession illegal. See Staples, supra.
B. Scope of Cross-Examination
Spinner contends that the district court improperly permitted the prosecutor to cross-examine his girlfriend, Lolita Little, about a letter Ms. Little wrote to Spinner two months before his arrest in this case. As we recounted above, the prosecutor asked Ms. Little if “last May” she had been “upset that Richard Spinner was letting his friend use his mother’s house to sell drugs.” The prosecutor based her question on a letter dated May 13, 1996 that Ms. Little had written to Spinner when he was incarcerated for an unrelated crime.
Without referring to Rule 404(b), the prosecutor attempted to justify her questions by arguing that they were meant to rebut the defense theory of the case. She referred to the defense’s opening statement in which, she claimed, counsel argued that Spinner “changed his life around” after his'brother died in August of 1995. The letter showed— according to the prosecutor — that “Mr. Spinner knew that [his cousin] was using, was selling drugs out of this premises” in May of 1996. In particular, the prosecutor argued that Spinner, while incarcerated, was “aiding and abetting” his cousin’s drug dealing at his mother’s house in May of 1996. Based on this evidence, the prosecutor argued that “Mr. Spinner was not a changed man.”
As we have noted, the government sought to use the letter to dispute that Spinner was a “changed man” who had “changed his life around” after his brother’s death. Using the letter to paint Spinner as “greedy,” and the kind of man who did nothing while “those m* * * * *f* * * * *s ... us[ed][his] mother and her home to do this s* * *,” the government clearly hoped to use the letter to convince the jury that Spinner had not “changed his life around”; it hoped, that is, to show that Spinner remained a criminal after his brother died. Using evidence for this purpose is forbidden by Rule 404(b) of the Federal Rules of Evidence, which provides that “evidence of other crimes, wrongs, or acts is not admissible to prove the character of a person in order to show action in conformity therewith.”
We recognize that under Rule 404(b), “any purpose for which bad-acts evidence is introduced is a proper purpose so long as the evidence is not offered solely to prove character.” United States v. Miller, 895 F.2d 1431, 1436 (D.C.Cir.1990). Rule 404(b) itself lists permissible uses of such evidence: “proof of motive, opportunity, intent, preparation, plan, knowledge, identity, or absence of mistake or accident.” In addition to arguing that the letter was intended to rebut the defense theory of the case— which, as we have explained, amounts to impermissible character evidence — the government also said that the statements in the letter were relevant to the witness’s credibility and bias. If the letter did undermine the witness’s credibility, the government could be said to have asked its questions for a reason other than to show Spinner’s character. See Miller, 895 F.2d at 1436 (using bad acts evidence to show character is “the one impermissible purpose for such evidence” under Rule 404(b)). Under these circumstances, Rule 404(b) would pose no barrier to admissibility. The government, however, has not satisfactorily explained how the letter served to impeach Ms. Little. The prosecutor asked Ms. Little: “around the same time that you wrote this letter, you were also upset that Richard Spinner was letting his friend use his mother’s house to sell drugs.” But Ms. Little had not testified to the contrary when the prosecutor asked that question. Furthermore, we do not understand how Ms. Little’s being upset under such circumstances would undermine her credibility at trial.
[961]*961Nor do we agree with the district court’s conclusion (not urged by the government) that the statements in the letter would be admissible to show “intent” or “absence of mistake” under Rule 404(b). In reaching this conclusion, the district court apparently agreed with the government that “do[ing] this s* * * ” meant drug dealing. But it seems just as likely to us that this reference is to using drugs as to selling them. Such a reading, we suppose, is equally consistent with the letter’s conclusion that Spinner would “end up locked up or dead” if he continued to do this “s* * Nor is it apparent that this reference is to crack cocaine, the narcotic at issue in this case. Nor is it clear that the reference is to narcotics at all. Under these circumstances, the statements in the letter have virtually no bearing on whether Spinner possessed crack cocaine with the intent to distribute it in this case.
Even if the government had questioned Ms. Little about the letter for a permissible purpose under Rule 404(b), it failed to provide “reasonable notice” to the defense of its intent to use the letter, as required by the rule. Rule 404(b) provides that “the prosecution in a criminal case shall provide reasonable notice in advance of trial, or during trial if the court excuses pretrial notice on good cause shown, of the general nature of any such evidence it intends to introduce at trial.” Here, the district court, mistakenly thinking that the government had asked the pertinent question in order to impeach a perjurer, ruled that “notice was not required” because “something [arose] in the defense ease that was unexpected....” But Rule 404(b) does not empower a district judge to excuse the government from providing any notice that it intends to use bad acts evidence. That rule requires the government to “provide reasonable notice ... during trial if the court excuses pretrial notice on good cause shown....” The government provided no notice to the defense, reasonable or otherwise, of its intention to question Ms. Little about the letter. Without approaching the bench, the prosecutor simply asked Ms. Little about Spinner’s refusal to stop his friends from using his mother’s house to sell drugs. On appeal, the government makes much of the fact that it had provided the letter to the defense during discovery. But providing such evidence to the defense in discovery is not enough to satisfy the notice requirements of Rule 404(b), which requires the government specifically to disclose “the general nature of any such evidence it intends to introduce at trial.”
For the reasons discussed above, we conclude that the district court abused its discretion when it permitted the government to question Ms. Little about the letter. See United States v. Graham, 83 F.3d 1466, 1473 (D.C.Cir.1996). Furthermore, we cannot conclude that the admission of this testimony was harmless error. See United States v. Clarke, 24 F.3d 257, 267 (D.C.Cir.1994). In deciding whether error is harmless, we must “determine whether the error itself had a substantial influence on the verdict.” Id. (internal quotation marks and citations omitted). In other words, “we must determine with fair assurance that the judgment was not substantially swayed by the error.” /(¿.(internal quotation marks, ellipses and citations omitted).
The prosecutor emphasized the letter in her closing argument to the jury, suggesting to us that the letter may have had a “substantial impact” on the verdict. After summarizing the evidence against Spinner, the prosecutor concluded her remarks concerning Spinner’s drug possession as follows:
[Drug dealing] is what Lolita Little was referring to in her letter as something that if he didn’t stop it he would get locked up or dead. And she suggested he had the ability to stop it. He had the ability to stop his friends from doing it. And he simply didn’t. And that, ladies and gentlemen, makes him guilty of possession with intent to distribute crack cocaine within a thousand feet of Davis Elementary School.
In addition, the government did not have an overwhelming case against Spinner for possession of the crack cocaine recovered in the house: no fingerprint evidence connected Spinner to the crack, nor was any of the crack recovered from his person or presence. Finally, we note that, although the district court gave the jury limiting instructions on other evidence introduced pursuant to Rule 404(b), it did not provide a similar limiting [962]*962instruction for the statements in Ms. Little’s letter, even though the court admitted them as “legitimate 404(b) evidence,” and characterized them as “clearly prejudicial” to Spinner. See United States v. Moore, 732 F.2d 983, 990 (D.C.Cir.1984) (limiting instruction lessens potential prejudice of bad act evidence).
Because we conclude that it was not harmless error for the district court to permit the government to use the letter in its cross-examination of Ms. Little, we reverse Spinner’s conviction for possession with intent to distribute cocaine within 1,000 feet of a school.
III. Conclusion
For the foregoing reasons, we reverse Spinner’s convictions for possession of a semiautomatic assault weapon and possession with intent to distribute cocaine within 1,000 feet of a school, and remand for resen-tencing. Spinner has raised additional arguments. We have given them full consideration, and determine that they warrant neither reversal of his remaining convictions nor discussion here. Accordingly, we affirm Spinner’s convictions for possession of a firearm and ammunition by a convicted felon.