Vuono, J.
A Superior Court jury convicted the defendant of [725]*725unlawful possession of a firearm (G. L. c. 269, § 10[a]) and unlawful possession of cocaine. After the jury returned their verdicts, the defendant pleaded guilty to the enhanced sentence portion of the firearm indictment. See G. L. c. 269, § 10G(a). The sole issue on appeal is whether a new trial is required as a result of the erroneous admission in evidence of a ballistics certificate and certificates of analysis for the cocaine, each of which came in without testimony of the certifying official. See Melendez-Diaz v. Massachusetts, 129 S. Ct. 2527 (2009) (Melendez-Diaz). For the reasons that follow, we affirm the defendant’s conviction of possession of a firearm and reverse the conviction of possession of cocaine.
Background. The charges in this case arose from a shooting incident on January 1, 2007, at a restaurant in Springfield. At approximately 2:00 a.m., Springfield police officers were dispatched to the restaurant to investigate a report of gunshots fired. When they arrived, the police observed two people on the ground and another person, later identified as the defendant, hobbling away from the scene.1 As the defendant was leaving, he discarded a handgun. The police quickly apprehended the defendant and later retrieved the gun, which had an empty magazine in it and no bullet in the chamber. Upon ascertaining that the defendant had been shot in the foot, the police transported him to the hospital where he received treatment for the wound. While he was at the hospital, medical personnel found two packets, one containing powder cocaine and the other containing “crack” cocaine, in the defendant’s sock.
At trial, the defendant did not contest that the gun in question was a firearm as defined in G. L. c. 140, § 121. He did not object to the admission of the ballistics certificate so identifying it during the Commonwealth’s case-in-chief. Instead, the defense was one of necessity.2 The defendant testified that after the shooting began he saw the gun on a table and “grabbed” it so it [726]*726could not be used against him or others. He claimed that he never fired the gun and did not know if the gun had been fired by someone else. To rebut the defendant’s testimony, the Commonwealth presented evidence from a ballistics expert, Trooper John Schrijn. Although Trooper Schrijn was not the officer who signed the ballistics certificate, he testified that he had examined the firearm and identified it as a nine millimeter caliber Jennings model semiautomatic pistol. He explained that during police testing the firearm “was fired by use of [police] ammunition ... for test specimens to be recovered from our water tank to be compared against . . . other recovered evidence in the [crime scene] area.” Trooper Schrijn examined spent cartridge casings retrieved from the crime scene and compared them to the casings obtained during the test firing of the firearm and “positively identified” two nine millimeter empty cartridge casings found outside the restaurant on the sidewalk as having been discharged by the firearm in question. He further testified that his examination at the scene disclosed that at least two other firearms had been fired inside the restaurant.
The defendant, through counsel, also conceded “the possession of the cocaine” (see note 2, supra), and he identified the substances as crack cocaine and powder cocaine during his testimony. However, unlike with the firearm charge, he presented no defense to the drug charge.3
Discussion. Although there was no objection, the applicable standard of review is whether the erroneous admission of the certificates of drug and ballistics analysis was harmless beyond a reasonable doubt. Commonwealth v. Vasquez, 456 Mass. 350, 360 [727]*727(2010)4 “[T]o establish harmlessness beyond a reasonable doubt, the Commonwealth must show that other properly admitted evidence of guilt is ‘overwhelming,’ in the sense that it is ‘so powerful as to “nullify any effect” ’ that the improperly admitted evidence ‘might have had’ on the fact finder or the findings.” Id. at 362, quoting from Commonwealth v. Tyree, 455 Mass. 676,704 n.44 (2010). In determining whether the Commonwealth has met its burden here, we do not consider the defendant’s admissions made during his testimony. See Commonwealth v. Mendes, 78 Mass. App. Ct. 474, 480-481 (2010), further appellate review granted, 459 Mass. 1104 (2011) (holding that it is not “appropriate” in the harmless error analysis to rely on admissions made by a defendant during his testimony at trial, because the court “cannot know whether the . . . testimony was a response to the erroneous admission of the certificates and hence tainted by the error”).5
1. Ballistics certificate. The Commonwealth introduced the erroneously admitted ballistics certificate as prima facie evidence that the pistol recovered by police met the statutory definition of a firearm, an essential element of the offense charged. See Commonwealth v. Pittman, 76 Mass. App. Ct. 905, 906 (2010). Despite the defendant’s concession at trial that he had “possess [ed] the gun” (see note 2, supra), the Commonwealth agrees that it retained the ultimate burden of proving at trial that the pistol was operable.6 It argues on appeal that other, lawfully admitted evidence of operability7 was potent enough to render the admission of the ballistics certificate harmless beyond a reasonable doubt. We agree.
[728]*728During its case-in-chief and on rebuttal, the Commonwealth presented significant evidence — independent of the ballistics certificate — that the pistol met the statutory definition of a firearm. In addition to the gun itself, which the Commonwealth introduced as an exhibit, the jury were shown security footage from the shooting depicting the discharge of multiple firearms, and heard testimony from officers that numerous cartridge casings, including two matching the pistol in evidence, were retrieved from the crime scene. Based on the evidence, the jury could have comfortably concluded that the subject firearm was among those depicted being fired. See Commonwealth v. Mendes, 75 Mass. App. Ct. 390,397 (2009) (independent evidence consisting of three audible shots, the recovery of three empty casings, and the smell of gunpowder rendered the admission of a ballistics certificate harmless beyond a reasonable doubt). Equally if not more compelling was Trooper Schrijn’s testimony in rebuttal that the gun had been test fired in order to obtain spent casings for comparison with evidence retrieved from the crime scene, and his opinion that two of the spent casings found at the scene resulted from shots fired by the firearm in question. This independent testimony overwhelmingly supported the inference that the gun was capable of firing a bullet. Accord Commonwealth v. Depina, 456 Mass. 238, 249-251 (2010). Taken in combination, the quantity and quality of the lawfully admitted evidence satisfies us that it was sufficient to “nullify any effect” the erroneously admitted ballistics certificate “might have had.” Vasquez, 456 Mass. at 362.
2. Certificates of drug analysis.
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Vuono, J.
A Superior Court jury convicted the defendant of [725]*725unlawful possession of a firearm (G. L. c. 269, § 10[a]) and unlawful possession of cocaine. After the jury returned their verdicts, the defendant pleaded guilty to the enhanced sentence portion of the firearm indictment. See G. L. c. 269, § 10G(a). The sole issue on appeal is whether a new trial is required as a result of the erroneous admission in evidence of a ballistics certificate and certificates of analysis for the cocaine, each of which came in without testimony of the certifying official. See Melendez-Diaz v. Massachusetts, 129 S. Ct. 2527 (2009) (Melendez-Diaz). For the reasons that follow, we affirm the defendant’s conviction of possession of a firearm and reverse the conviction of possession of cocaine.
Background. The charges in this case arose from a shooting incident on January 1, 2007, at a restaurant in Springfield. At approximately 2:00 a.m., Springfield police officers were dispatched to the restaurant to investigate a report of gunshots fired. When they arrived, the police observed two people on the ground and another person, later identified as the defendant, hobbling away from the scene.1 As the defendant was leaving, he discarded a handgun. The police quickly apprehended the defendant and later retrieved the gun, which had an empty magazine in it and no bullet in the chamber. Upon ascertaining that the defendant had been shot in the foot, the police transported him to the hospital where he received treatment for the wound. While he was at the hospital, medical personnel found two packets, one containing powder cocaine and the other containing “crack” cocaine, in the defendant’s sock.
At trial, the defendant did not contest that the gun in question was a firearm as defined in G. L. c. 140, § 121. He did not object to the admission of the ballistics certificate so identifying it during the Commonwealth’s case-in-chief. Instead, the defense was one of necessity.2 The defendant testified that after the shooting began he saw the gun on a table and “grabbed” it so it [726]*726could not be used against him or others. He claimed that he never fired the gun and did not know if the gun had been fired by someone else. To rebut the defendant’s testimony, the Commonwealth presented evidence from a ballistics expert, Trooper John Schrijn. Although Trooper Schrijn was not the officer who signed the ballistics certificate, he testified that he had examined the firearm and identified it as a nine millimeter caliber Jennings model semiautomatic pistol. He explained that during police testing the firearm “was fired by use of [police] ammunition ... for test specimens to be recovered from our water tank to be compared against . . . other recovered evidence in the [crime scene] area.” Trooper Schrijn examined spent cartridge casings retrieved from the crime scene and compared them to the casings obtained during the test firing of the firearm and “positively identified” two nine millimeter empty cartridge casings found outside the restaurant on the sidewalk as having been discharged by the firearm in question. He further testified that his examination at the scene disclosed that at least two other firearms had been fired inside the restaurant.
The defendant, through counsel, also conceded “the possession of the cocaine” (see note 2, supra), and he identified the substances as crack cocaine and powder cocaine during his testimony. However, unlike with the firearm charge, he presented no defense to the drug charge.3
Discussion. Although there was no objection, the applicable standard of review is whether the erroneous admission of the certificates of drug and ballistics analysis was harmless beyond a reasonable doubt. Commonwealth v. Vasquez, 456 Mass. 350, 360 [727]*727(2010)4 “[T]o establish harmlessness beyond a reasonable doubt, the Commonwealth must show that other properly admitted evidence of guilt is ‘overwhelming,’ in the sense that it is ‘so powerful as to “nullify any effect” ’ that the improperly admitted evidence ‘might have had’ on the fact finder or the findings.” Id. at 362, quoting from Commonwealth v. Tyree, 455 Mass. 676,704 n.44 (2010). In determining whether the Commonwealth has met its burden here, we do not consider the defendant’s admissions made during his testimony. See Commonwealth v. Mendes, 78 Mass. App. Ct. 474, 480-481 (2010), further appellate review granted, 459 Mass. 1104 (2011) (holding that it is not “appropriate” in the harmless error analysis to rely on admissions made by a defendant during his testimony at trial, because the court “cannot know whether the . . . testimony was a response to the erroneous admission of the certificates and hence tainted by the error”).5
1. Ballistics certificate. The Commonwealth introduced the erroneously admitted ballistics certificate as prima facie evidence that the pistol recovered by police met the statutory definition of a firearm, an essential element of the offense charged. See Commonwealth v. Pittman, 76 Mass. App. Ct. 905, 906 (2010). Despite the defendant’s concession at trial that he had “possess [ed] the gun” (see note 2, supra), the Commonwealth agrees that it retained the ultimate burden of proving at trial that the pistol was operable.6 It argues on appeal that other, lawfully admitted evidence of operability7 was potent enough to render the admission of the ballistics certificate harmless beyond a reasonable doubt. We agree.
[728]*728During its case-in-chief and on rebuttal, the Commonwealth presented significant evidence — independent of the ballistics certificate — that the pistol met the statutory definition of a firearm. In addition to the gun itself, which the Commonwealth introduced as an exhibit, the jury were shown security footage from the shooting depicting the discharge of multiple firearms, and heard testimony from officers that numerous cartridge casings, including two matching the pistol in evidence, were retrieved from the crime scene. Based on the evidence, the jury could have comfortably concluded that the subject firearm was among those depicted being fired. See Commonwealth v. Mendes, 75 Mass. App. Ct. 390,397 (2009) (independent evidence consisting of three audible shots, the recovery of three empty casings, and the smell of gunpowder rendered the admission of a ballistics certificate harmless beyond a reasonable doubt). Equally if not more compelling was Trooper Schrijn’s testimony in rebuttal that the gun had been test fired in order to obtain spent casings for comparison with evidence retrieved from the crime scene, and his opinion that two of the spent casings found at the scene resulted from shots fired by the firearm in question. This independent testimony overwhelmingly supported the inference that the gun was capable of firing a bullet. Accord Commonwealth v. Depina, 456 Mass. 238, 249-251 (2010). Taken in combination, the quantity and quality of the lawfully admitted evidence satisfies us that it was sufficient to “nullify any effect” the erroneously admitted ballistics certificate “might have had.” Vasquez, 456 Mass. at 362.
2. Certificates of drug analysis. Like the ballistics certificate discussed above, the Commonwealth introduced the erroneously admitted certificates of drug analysis to prove an essential element of a charged offense, the chemical composition of the alleged cocaine. In contrast to the firearm evidence, however, the record contains virtually no evidence, aside from the certificates and the defendant’s testimonial admissions, from which the jury independently could have inferred that the substances found in the defendant’s sock were cocaine. The arresting officer’s conclu-sory observation that the substances retrieved from the defendant’s sock at the hospital “appeared to be” crack and powder cocaine, upon which the Commonwealth principally relies, is [729]*729insufficient to show harmlessness beyond a reasonable doubt. See Vasquez, 456 Mass. at 365; Commonwealth v. Charles, 456 Mass. 378, 382 (2010). This court’s recent decision in Mendes, 78 Mass. App. Ct. at 480-481, does not permit us to consider the defendant’s testimonial admissions as substantive evidence of guilt in the harmless error analysis.8 Accordingly, we are constrained to vacate the defendant’s conviction of possession of cocaine.
The dissent posits that, despite the lack of independent evidence establishing the chemical composition of the alleged cocaine, we may affirm the defendant’s drug conviction based on his concession at trial to “possession of the cocaine” and on the judge’s communication of that concession to the jury. This concession, the dissent reasons, was part of an “obvious trial strategy ... to give a little [the cocaine charge for which the defendant was facing a relatively short sentence] in the hope of gaining a lot [credibility in the eyes of the jury for his necessity defense on the more serious gun charge].” Even assuming this to be true, and although such a strategy might have been reasonable in the circumstances, our cases make clear that the “defendant’s theory of his case cannot relieve the Commonwealth of its burden of proving every element of a crime beyond a reasonable doubt.” Charles, supra at 383, quoting from Commonwealth v. Shea, 398 Mass. 264, 269 (1986). “[B]y giving the defendant the benefit of the harmless beyond a reasonable doubt standard, as we have, ‘the defendant is effectively in the same position as if he had objected to the admission of the drug certificates.’ ” Charles, supra, quoting from Vasquez, 456 Mass. at 368. Ultimately, the Commonwealth must show that admissible evidence independent of the certificates overwhelmingly proves the nature of the charged substances.
The dissent also argues that the defendant’s concession amounted to a “stipulation,” which relieved the government of its burden of proving the composition of the charged substances by lawfully admitted evidence. In conducting harmless error analysis under Melendez-Diaz, our courts have often noted that [730]*730the Commonwealth retains the burden of proving each element of a crime charged absent a “stipulation.” See, e.g., Commonwealth v. Muniz, 456 Mass. 166, 173 n.7 (2010); Vasquez, supra at 367-368, citing Melendez-Diaz, 129 S. Ct. at 2542. Although our cases do not precisely delimit what constitutes a “stipulation” in this context, the Supreme Judicial Court has held that the government retains its burden of proof in a drug case where a defendant does not specifically “stipulate to the composition of the substances” (emphasis supplied). Charles, 456 Mass. at 383 (rejecting the argument that defense counsel may “tacitly stipulate)]” to the nature of a substance by repeatedly referring to it as “cocaine” at trial). Here, as in Charles, counsel’s representation that the defendant was “conced[ing] . . . possession of the cocaine” fails to stipulate to, or even specifically address, the composition of the substance. We, therefore, decline to treat it as relieving the Commonwealth of its burden on this element.9
Conclusion. For the foregoing reasons, we reverse the judg[731]*731ment on the indictment charging possession of cocaine, set aside the verdict, and remand the case to the Superior Court for further proceedings. The judgment on the indictment charging unlawful possession of a firearm is affirmed.
So ordered.