ARMSTRONG, J.

The defendants, Darden and Ferriabough, were indicted jointly for conspiracy to distribute cocaine and separately for armed robbery. They were convicted of those offenses and have appealed under G.L.c. 278, §§ 33A-33G. The evidence taken in the light most favorable to the Commonwealth warranted the jury's finding the following to be the facts which supported the convictions.

On September 29, 1975, one Dooling, a police informant, who had previously witnessed Ferriabough obtaining drugs for an addict, and who had been called on the telephone by Ferriabough to see if he wanted to buy an ounce of cocaine for $1,200, met Ferriabough near a bar in the vicinity of LaGrange Street in Boston. Darden was with Ferriabough. The defendants said they were prepared to sell him the ounce of cocaine. Dooling stated that he had been unsuccessful in obtaining the money for the purchase, which angered Ferriabough. Dooling was permitted to test the cocaine. While waiting to test the cocaine, he saw Darden, and later, Ferriabough, talking with one Mayo, who had been in the bar. Dooling and the defendants arranged a meeting for the following day for the purpose of completing the transaction. Dooling informed Detective Maher of the Boston police drug control unit of the impending sale.

On September 30, Dooling met at Boston police headquarters with Detective Maher and one Stephen Murphy, an agent of the Drug Enforcement Administration (DEA) of the United States Department of Justice. Fifteen hundred dollars in marked bills was hidden in the trunk of Dooling's car. Murphy and Dooling then drove the car to the arranged meeting place, a club in Park Square. They were followed surreptitiously by a number of DEA agents and Boston police officers, including Detective Maher. Darden insisted that the transaction take place in another location, saying that he had to be careful because he had just completed serving a Federal prison term for narcotics violations. The place chosen was on Huntington Avenue, in front of the Young Men's Christian Association. Ferriabough joined Murphy and Dooling in Dooling's car; Darden took a taxicab to pick up Mayo, who (according to Darden) had the cocaine. The Dooling car arrived first. When the taxicab came, Darden got out, got into Dooling's car, and directed Murphy, who was driving, to a place near the end of Gainsborough Street, where a footbridge crosses railroad tracks. Murphy parked the car some distance away. Darden got out, walked to the end of Gainsborough Street, and entered the waiting taxicab. Mayo got out of the taxicab and waited. Murphy and Ferriabough walked to the end of Gainsborough Street and met Mayo, who said he would hand over the cocaine on the footbridge. Darden, seated in the taxicab, held a silver shopping bag on his lap. As Murphy and Ferriabough walked towards the footbridge, Mayo went to the taxicab, took the silver shopping bag, and followed the other two. He sat on the stairs of the footbridge, waited for a woman and children to pass, then reached into the shopping bag and pulled out a sawed-off shotgun. He aimed it at Murphy's stomach. Ferriabough laughed and took the car keys from Murphy. He returned to the Dooling car, opened the trunk where he knew the money was hidden, removed it, and started to flee. Meanwhile, Mayo had ordered Murphy to walk across the footbridge, threatening, when Murphy feigned illness, "to kill [him] right there." At some point Murphy drew a hand gun from his waistband, and a gunfight ensued in which Mayo was mortally wounded. DEA agents and Boston police officers converged on the scene. Darden, Ferriabough and the taxi driver were all arrested and taken to Boston police headquarters. The time was then approximately 2:30 P.M. At 5:20 P.M., Ferriabough made a statement to the police which was taped and which amounted to a confession on the conspiracy charge. It also confirmed many of the facts stated above concerning the armed robbery charge but portrayed the defendants as surprised by Mayo's possession and use of the gun. The statement was read in evidence during the cross-examination of Ferriabough.

1. The defendants contend that the judge erred in denying Ferriabough's pre-trial motion to suppress his statement and in allowing it in evidence. At the voir dire the defendants sought to show that the statement was obtained by threats and coercion and that the police denied the defendants their rights under G.L.c. 276, § 33A, to make telephone calls within an hour after their arrests. The evidence adduced by the defendants to substantiate the claim of threats and coercion was not consistent (the taxi driver's testimony differed substantially from the defendants') and was explicitly denied by each of the law enforcement officers who testified. By his denial of the motion the judge impliedly accepted the testimony of the latter.

We may assume that the judge, in the absence of evidence to the contrary from the police, credited the testimony of the defendants and the cabby that they were not informed of their right to use the telephone nor permitted use thereof for approximately two hours after their arrival at the police station. Those omissions, even if not intentional, would violate the provisions of § 33A and would require suppression of any evidence obtained as a result thereof. Commonwealth v. Bouchard, 347 Mass. 418, 419-421 (1964). Commonwealth v. Jones, 362 Mass. 497, 502 (1972). If such an omission is intentional, an in-custody inculpatory statement is inadmissible unless the Commonwealth proves "beyond a reasonable doubt that the evidence is untainted by the deprivation of the defendant's right." Id. at 503.

That burden was met in this case. The evidence was uncontradicted that the defendants were fully informed, not only of their Miranda rights, but also of their present right to use the telephone, before the start of the interrogation which resulted in Ferriabough's statement. From all the evidence at the voir dire it appears that the delay was caused by the fact that Detective Maher, who was in charge of the case, was delayed for more than two hours at the scene of the arrests, very likely due to the unexpected homicide, and that until his arrival at the station the defendants were simply made to wait. Upon his arrival, all proper procedures were followed, and no evidence was obtained, or sought to be obtained, from either defendant prior to Detective Maher's arrival. The judge did not err in denying the motion to suppress the statement Ferriabough made thereafter.

2. The defendants argue that the judge erred in permitting the Commonwealth to use a prior consistent statement relating to two significant portions of Murphy's testimony. The ruling was made in the following context.

Murphy had testified on direct examination (1) that Darden carried on his lap in the taxicab the silver shopping bag from which Mayo later pulled the sawed-off shotgun, and (2) that Ferriabough laughed when Mayo pulled out the gun and aimed it at Murphy's stomach. Those details of Murphy's testimony were of significance in the Commonwealth's case because of the defendants' position, advanced in Ferriabough's confession and again in his testimony at trial, that he and Darden were unaware that Mayo was carrying a gun, and that they had no intention of robbing Murphy.

In cross-examination the defendants attempted to impeach Murphy's testimony in those respects through the use of a transcribed statement which Murphy had made to officers of the homicide division of the Boston police a few hours after the shooting incident and which made no mention of either of the disputed facts. On redirect examination the Commonwealth was permitted, over objection and exception, to show that Murphy had made mention of both facts in the written report which Murphy had filed with his own agency six days after the incident. It could be found, on Murphy's testimony, that Murphy had actually written the report the morning after the incident.

The general rule is well established, that a witness's testimony at trial may not be bolstered or corroborated by evidence that he said the same thing on prior occasions. Commonwealth v. Tucker, 189 Mass. 457, 484 (1905). Commonwealth v. Retkovitz, 222 Mass. 245, 249-250 (1915). Wilson v. Jeffrey, 328 Mass. 192, 194 (1951). Commonwealth v. Zukoski, 370 Mass. 23, 27 (1976). See generally 4 Wigmore, Evidence §§ 1122-1133 (Chadbourn rev. 1972). But it is also well settled that when a witness's testimony at trial is impeached by a prior statement, the witness must be permitted to explain the prior statement and the reason for any omission or inconsistency. Commonwealth v. Smith, 329 Mass. 477 (1952). Commonwealth v. Fatalo, 345 Mass. 85 (1962). Leach Liacos, Massachusetts Evidence 130 (1967). 3A Wigmore, Evidence § 1044 (Chadbourn rev. 1970). 2 Torcia, Wharton's Criminal Evidence § 499 (13th ed. 1972). If the prior statement suggests that the witness's testimony is of recent contrivance, a prior consistent statement may be shown as part of such an explanation provided that it has probative value: i.e., provided that it has a logical tendency to establish the truth of the explanation and rebut the appearance of contrivance. Hewitt v. Corey, 150 Mass. 445 (1890). See also Openshaw v. Adams, 92 Idaho 488, 493-494 (1968); People v. Gentry, 270 Cal.App.2d 462, 473 (1969); Donovan v. Moore-McCormack Lines, Inc. 266 App. Div. 406 (N.Y. 1943); 2 Torcia, Wharton's Criminal Evidence § 500, at 495 (13th ed. 1972). Such use of prior consistent statements should be allowed only with caution, and where the probative value for the proper purpose is clear, because of the ever present danger that the jury will, despite instructions, consider the prior consistent statement as evidence of the facts therein asserted. Commonwealth v. Tucker, supra, at 483-484. Boutillette v. Robbins, 338 Mass. 195, 197-198 (1958).

The prior consistent statements made in Murphy's DEA report satisfied the prerequisites for admissibility. The thrust of the attempted impeachment was to suggest to the jury that, if Murphy's assertions concerning Darden's possession of the silver shopping bag containing the shotgun and Ferriabough's reaction to Mayo's use of the gun against Murphy were true, Murphy would have mentioned them in an official report made to the police shortly after the event when his memory of events was fresh; and the showing of his failure to do so was properly found by the judge (see Commonwealth v. Zukoski, 370 Mass. at 25-26) to have suggested to the jury that those assertions were embellishments or contrivances intended to strengthen the Commonwealth's case against the defendants. In these circumstances it was proper for the Commonwealth to suggest to the jury that the Boston police report, by which Murphy was being impeached, was not in fact his official report but was a series of responses to questions put by homicide bureau police whose main interest lay in recording the circumstances of the homicide rather than in marshaling for trial evidence of the defendants' crimes; and that in preparing his official investigation report to his own agency, Murphy did in fact include many details of those crimes not mentioned in the homicide report, including the facts in question. The evidence had a logical tendency to explain the omission and rebut the attempted impeachment. Because (as could be found) the DEA report was made substantially contemporaneously with the Boston police report, it lay within the discretion of the judge to conclude that it had sufficient probative value in rebutting the attempted impeachment to warrant its admission in evidence. Compare State v. Neiman, 123 N.J.L. 341 (1939); 2 Torcia, Wharton's Criminal Evidence § 500, at 492-495 (13th ed. 1972).

The defendant Darden argues that Murphy's prior consistent statements in the DEA report were inadmissible because they were made after his statements recorded in the Boston police homicide report. Darden cites Commonwealth v. Zukoski, supra, at 27, for the proposition that a prior consistent statement may never be used to rebut an inference of recent contrivance based on a statement made prior thereto. We think that the defendant reads the cited passage in Commonwealth v. Zukoski incorrectly. In that case the prior consistent statement offered to rehabilitate the defendant's trial testimony lacked probative value because, in contrast to the two inconsistent statements made to the police immediately after the homicide, it was made substantially later, when the defendant was preparing his defense. We do not read that case as having laid down a hard and fast rule of priority, and we decline to adopt such a rule ourselves. See 4 Wigmore, Evidence § 1126 at 267 (Chadbourn rev. 1972). Cf. Hewitt v. Corey, 150 Mass. 445 (1890). Rather, the test should remain one of probative value — whether the prior consistent statement has a logical tendency to meet and counter the suggestion that the witness has recently contrived his testimony for purposes of trial. In making that determination, a measure of discretion must be given to the trial judge (compare Commonwealth v. Zukoski at 27, and cases cited), who is in a better position than an appellate court to evaluate the subtleties and nuances of the trial and who consequently should not be reversed for admitting or excluding such testimony except where it is clearly shown that his ruling is outside the proper boundaries of discretion. Such a showing has not been made in this case.

3. The trial judge denied Ferriabough's request for a special instruction to the jury relative to assessing the credibility of his testimony. Assuming, without deciding, that the requested instruction was sufficiently balanced that the judge might properly have given it in the form submitted, we think it nevertheless lay within the judge's discretion not to give it. Compare Commonwealth v. Greenberg, 339 Mass. 557, 585 (1959). He may well have concluded "that the jury in the exercise of common sense and experience do not need to be told much about this matter." Commonwealth v. Beal, 314 Mass. 210, 232 (1943). The judge's instructions to the jury explained the presumption of innocence and the heavy burden of proof borne by the Commonwealth. He explained that in grand jury proceedings "the defendants have no right to introduce evidence, . . . cannot introduce evidence [, a]nd . . . do not have any means of defending themselves before the grand jury." In the face of these instructions it cannot reasonably be contended that the jury were left with the impression that they could reject Ferriabough's testimony solely because he was on trial. We have considered the cases from other jurisdictions cited by Ferriabough for the proposition that a trial judge in a criminal case must give such a special instruction if requested, as well as cases cited by the Commonwealth to the contrary. Our law has permitted defendants to testify in their own behalf in criminal cases for well over a century (see St. 1866, c. 260, now G.L.c. 233, § 20, Third); and the very absence in our appellate reports since that time of any contention that a judge must give a special instruction concerning the weight to be given a defendant's testimony (compare Commonwealth v. Wright, 107 Mass. 403) seems to us to suggest rather strongly that the defense bar has felt that little purpose would be served by encumbering jury charges with such an instruction.

4. The contention of the defendant Darden that the trial judge abused his discretion in denying a continuance on the final day of the trial to enable Darden to search for and subpoena a witness, one Al Franklin, has no merit whatever: Franklin was the person named by Dooling as the drug addict for whom Ferriabough had procured heroin prior to the events of September 29 and 30 — a story confirmed, incidentally, in Ferriabough's testimony. The transcript does not bear out Darden's contention that his request for the continuance was timely made in the circumstances; more important, it is clear that the purpose for which Franklin's testimony was sought was of only the most marginal significance in the total context of the trial. Under the test set out in Commonwealth v. Gilchrest, 364 Mass. 272, 276-277 (1973), the denial of the request was well within the judge's discretion. Compare Commonwealth v. Nassar, 354 Mass. 249, 264 (1968), cert. den. 393 U.S. 1039 (1969); Commonwealth v. Bettencourt, 361 Mass. 515, 517-518 (1972); Commonwealth v. LaFleur, 1 Mass. App. Ct. 327, 330 (1973); Commonwealth v. Dominico, 1 Mass. App. Ct. 693, 705-706 (1974).

Judgments affirmed.