John Keith McCauley appeals from his conviction following a jury trial for possession of an unregistered firearm in violation of 26 U.S.C. § 5861(d) (1976).1 On appeal, McCauley contends that: (1) 26 U.S.C. § 5845(b), defining “machinegun” for the purposes of the National Firearms Act (Act), is unconstitutional as applied in this case; (2) the trial court erred in admitting into evidence a tape recording of a conversation between McCauley, his brother-in-law, and an undercover agent of the Bureau of Alcohol, Tobacco and Firearms (BATF); (3) the trial court erroneously allowed the Government’s expert witness to give his opinion that McCauley’s weapon was a ma-chinegun covered by the Act; and (4) the trial court erred in admitting into evidence a BATF registry certificate which contained a statement that McCauley had neither applied to transfer any firearms nor paid a transfer tax on any firearms. We *338reject these arguments and affirm the conviction.
1. Factual Background.
On July 3, 1978, McCauley possessed an unregistered Japanese type-96 light machi-negun, which lacked the magazine necessary for automatic firing. On that date, McCauley met with Darold Hays, his brother-in-law, and Victor J. Herbert, Jr., a BATF agent, at McCauley’s home in Ken-nett, Missouri, and agreed to sell the weapon to Herbert. BATF agents recorded that conversation and sale, by means of hidden transmitters voluntarily carried by both Hays and Herbert, and arrested McCauley immediately after the sale.
At trial McCauley conceded possession of the weapon. He contended, however, that he was under no duty to register the gun because without a magazine it could not be fired automatically, and, therefore, it was not a “machinegun” within the meaning of the registration statute.2 McCauley also introduced expert testimony that magazines for type-96 machineguns are rare and practically impossible to acquire on the open market.
The Government, on the other hand, introduced evidence that a type-96 machine-gun magazine was available at a St. Louis gun store and that, for a price of $45, a gunsmith in the St. Louis area had converted a German MG 13 magazine, which is readily obtainable on the open market, into a magazine useable in a type-96 machine-gun. In addition, the Government showed that, in offering to sell the type-96 machi-negun, McCauley told Herbert that he possessed a thirty-round “clip” (/. e., magazine) for the gun.
II. Discussion.
A. Definition of “Machinegun.”
McCauley initially contends that one of the definitions of “machinegun” in 26 U.S.C. § 5845(b),3 i. e., a weapon “designed to shoot * * * automatically more than one shot,” is unconstitutionally vague and overbroad for a strict liability statute such as the National Firearms Act. We need not address this contention here because the record establishes beyond question that the gun in McCauley’s possession met the alternative test in section 5845(b), as a weapon that “can be readily restored to shoot” automatically.
B. Admissibility of the Recorded Conversation.
The district court admitted into evidence the entire tape recording of the conversation between McCauley, Hays, and BATF agent Herbert, at which the sale of the machinegun was discussed. McCauley challenges the admissibility of this recording on three grounds: (1) the recording of the conversation constituted an unreasonable search and seizure; (2) the identification at trial of the recorded voices was inadequate; and (3) the prejudicial effect of the tape recording substantially outweighed its probative value. We reject each of these contentions.
First, no unreasonable search or seizure occurred here, because both Hays and Herbert, participants in the conversation, consented to the recording. United States v. White, 401 U.S. 745, 91 S.Ct. 1122, 28 L.Ed.2d 453 (1971).
*339Second, the voices on the tape were sufficiently clearly identified at trial to prevent confusion by the jury, particularly in light of Herbert’s testimony relating the substance of the crucial portions of the recorded conversation.
McCauley’s third objection to the admission of the entire tape recording, that the playing of the recording unfairly prejudiced him because it contained vulgarity, racial slurs, and cavalier statements concerning past gun sales, presents a more serious question. The better- practice might have been to admit only those limited portions of the recording in which McCauley discussed the machinegun in question. However, any error in admitting other portions of the recording was harmless given the overwhelming evidence of McCauley’s guilt of the crime charged.
C. Opinion Concerning an Ultimate Issue.
McCauley argues that the trial court erred in permitting the Government’s expert witness to testify that, in his opinion, McCauley’s weapon was a machinegun required to be registered under the Act. We find no abuse of discretion in the admission of this testimony; Under Fed.R.Evid. 704, such opinion testimony “is not objectionable because it embraces an ultimate issue to be decided by the trier of fact.”
D. The Firearms Registry Certificate.
Finally, McCauley contends that the trial court erred in admitting into evidence a BATF registry certificate indicating that McCauley had not registered any firearms and that he had neither applied to transfer a firearm nor paid a transfer tax on any firearm. McCauley maintains that the latter part of the certificate constitutes evidence of other crimes, namely, that he sold the machinegun to Herbert without applying to transfer it or paying a transfer tax as required by 26 U.S.C. § 5812(a).
While the certified statement contained information in addition to that required to support the indictment for possession of an unregistered firearm, that document was nonetheless relevant and admissible to establish that McCauley possessed a machinegun not registered to him in the National Firearms Registration and Transfer Record. See United States v. Stevens, 509 F.2d 683, 685 n.1(b) (8th Cir.), cert. denied, 421 U.S. 989, 95 S.Ct. 1993, 44 L.Ed.2d 479 (1975).
t7] The district court, at McCauley’s request, might have deleted the statements in the certificate pertaining to transfer, but McCauley failed to make such a request and, in any event, no prejudice is shown. The certificate, including the statements that McCauley had not applied to transfer any firearm or paid transfer tax upon any firearm, merely sets forth the national firearms registration and transfer record of McCauley. Those statements on the certificate, standing alone, do not show that McCauley has committed any crime. Even if the reception of the surplus material in the certificate was erroneous, such error must be deemed harmless in light of the overwhelming evidence in the record of McCauley’s guilt of the possession offense.
Affirmed.
. The Honorable James H. Meredith, Chief Judge, United States District Court for the Eastern District of Missouri, sentenced McCau-ley to four years’ imprisonment.
. 26 U.S.C. § 5861(d) makes it unlawful for any person “to receive or possess a firearm which is not registered to him in the National Firearms Registration and Transfer Record[.]” The definition of “firearm” in 26 U.S.C. § 5845(a) includes “a machinegun,” which § 5845(b) defines as
any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The terms shall also include the frame or receiver of any such weapon, any combination of parts designed and intended for use in converting a weapon into a machi-negun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.
In its jury instructions, the district court included only the first sentence of § 5845(b) in its definition of a machinegun.
. See note 2 supra.