Following a jury trial in the Superior Court, the defendant was convicted of assault and battery by means of a dangerous weapon, G. L. c. 265, § 15A, among other offenses.1 The Commonwealth’s case relied almost exclusively on the identification of the defendant by one of the two victims of the drive-by shooting out of which the charges arose; the theory of the defense was mistaken identification and alibi. In a motion for a new trial, the defendant contended that his trial counsel was ineffective in failing to present to the jury evidence that his appearance on the day of the incident was dramatically different from that depicted in the photograph selected by the identifying witness from a photographic array prepared by police. We vacate the order denying the defendant’s new trial motion and remand the case for an evidentiary hearing.
Background. At approximately 11:30 p.m. on December 20, 2002, Lawrence Akers was driving a car through downtown Springfield, on the way to a nightclub. Prez Cope and Mildred Rodriguez were passengers in the car, Cope in the front passenger seat and Rodriguez in the rear seat directly behind Cope. While waiting at a red light, Akers noticed a car pulling up next to him on the left. Though Akers could not see clearly into the other vehicle because the windows were tinted, he could see the silhouettes of multiple passengers in the other car, and it appeared to him that the front passenger was gesturing to him.
The traffic light turned green. As Akers pulled away from the stop, he and the passenger in the other vehicle simultaneously rolled down their windows. At the same time, Akers reached to his right for the volume control on his car radio, in order to turn it down. He then heard gun shots. Akers saw the face of the front passenger and saw a flash coming from what he thought was a gun in the passenger’s hand; Cope saw flashes, but did not see the passenger’s face. The exchange lasted a few seconds. Both Akers and Cope were struck by bullets in their legs.
*520As Akers drove to a nearby hospital, he ran a red light and was stopped by police. In response to police questioning during the stop, none of the occupants of Akers’s vehicle was able to identify the shooter or any of the other occupants of the vehicle from which the shots were fired, describing them only as “three light-skinned black males, all very young looking.”2 Neither were they able to furnish a license plate number or other specific identifying description for the vehicle; they described a dark-colored car, possibly a Nissan or Honda, with tinted windows. Later, at the hospital, Akers and Cope discussed the shooting with each other. Akers commented that the shooter looked like someone he knew, Leonard Naylor, but younger. Cope responded that Leonard Naylor had a younger brother named Adam, whom Cope knew from having played basketball with him. Akers did not know Adam Naylor, however.
The following day, Akers went to the police station. According to Detective Kervick, during preliminary questioning Akers mentioned that the shooter “looked like a Naylor that he went to school with that was younger.”3 Detective Kervick assembled a photographic array consisting of eight photographs, including a photograph of the defendant taken approximately two years before the shooting incident.4 All of the photographs in the array were of young black males, with short hair and no facial hair. Akers selected the defendant’s photograph from the array. Cope and Rodriguez, presented with arrays that included the same picture of the defendant, did not identify any of the photographs in the array as depicting the shooter.
Trial began on October 12, 2004, and concluded on October 14.5 At the time of trial, the defendant wore his hair braided, at shoulder length. He also wore a mustache and beard. According to an affidavit of the defendant submitted with his new trial motion, he wore his hair in the same manner in December, 2002, *521at the time of the shooting incident; that description is corroborated by a photograph of the defendant taken on November 11, 2002, also submitted with the new trial motion.* 6 As we have observed, the defense at trial was mistaken identification and alibi.7 8Specifically with reference to the theory of mistaken identification, trial counsel elicited testimony that the shooter had no facial hair and that the photograph of the defendant included in the array had been taken two years before the shooting incident. However, trial counsel did not elicit any testimony from Akers as to the shooter’s hair length, or from defense witnesses concerning the defendant’s shoulder-length braids. Neither did trial counsel direct the jury’s attention to the dramatic difference between the defendant’s appearance at the time of trial and his appearance in the photograph Akers selected from the array.
Discussion. “A defendant seeking a new trial based on a claim of ineffective assistance of counsel bears the burden to establish both prongs of the familiar test articulated in Commonwealth v. Saferian, 366 Mass. 89, 96 (1974): (i) ‘whether there has been serious incompetency, inefficiency, or inattention of counsel — behavior of counsel falling measurably below that which might be expected from an ordinary fallible lawyer’; and (ii) ‘whether [such ineffectiveness] has likely deprived the defendant of an otherwise available, substantial ground of defence.’ ” Commonwealth v. Pike, 53 Mass. App. Ct. 757, 760 (2002). On appeal from an order either allowing or denying a motion for a new trial, we review the order for abuse of discretion. See Commonwealth v. Grace, 397 Mass. 303, 307 (1986). Though we extend special deference to the action of a motion judge who was also the trial judge, where (as here) the motion judge did not preside at trial, “we regard ourselves in as good a position as the motion judge to assess the trial record.” Ibid. 8
*522As we have observed, the defense at trial was based on mistaken identification and alibi. In furtherance of that defense, the defendant’s trial counsel pointed out to the jury that the photograph included as part of the array reviewed by Akers was two years old at the time it was presented to him (so that the defendant was two years younger in the photograph than he was on the date of the shooting). Counsel also observed the many other factors detracting from the reliability of Akers’s identification: the brief duration of the encounter, the fact that Akers was driving forward and adjusting the radio volume at the time he and the shooter rolled down their car windows (and that the tinted windows of the other vehicle prevented Akers from seeing the shooter before the windows were rolled down), the dark nighttime conditions at the time of the encounter, the fact that Akers did not previously know the defendant, the fact that Cope (who did know the defendant) did not identify him as the shooter, and the fact that Akers’s attention was drawn to the defendant as a result of Cope’s comment that Leonard Naylor (who Akers thought resembled the shooter) had a younger brother.
However, trial counsel inexplicably failed to exploit what was perhaps the most significant basis available for the defense theory: the dramatic difference between the defendant’s appearance on the date of the shooting and his appearance in the photograph Akers selected from the array assembled by the police.9 There appear to have been no tactical grounds for trial counsel to eschew that line of attack on Akers’s identification of the defendant.10 The fact that the defendant had shoulder-length, braided *523hair on the date of the shooting, as he did at trial, could have been developed without adverse effect through testimony of the alibi witnesses (who included one of the defendant’s younger brothers).11 Moreover, counsel could have sought, through cross-examination of Akers, to elicit from Akers himself that he had not observed that the shooter had long hair or braids. Similarly, trial counsel did not cross-examine Cope at trial on the subject of the shooter’s hair. It appears that such cross-examination would have borne fruit: though Cope testified that he was unable to identify who the shooter was, Cope stated in an affidavit submitted with the defendant’s new trial motion that he saw the shooter and that the shooter did not have braids.12
There can be little doubt that counsel’s failure deprived the defendant of a substantial ground of defense. “Eyewitness identification of a person whom the witness had never seen before the crime or other incident presents a substantial risk of misidentification and increases the chance of a conviction of an innocent defendant.” Commonwealth v. Jones, 423 Mass. 99, 109 (1996). See Commonwealth v. Johnson, 420 Mass. 458, 466 (1995), quoting from United States v. Wade, 388 U.S. 218, 228 (1967) (“the vagaries of eyewitness identification are well-known; the annals of criminal law are rife with instances of mistaken identification”). “Indeed, studies conducted by psychologists and legal researchers . . . have confirmed that eyewitness *524testimony is often hopelessly unreliable.” Commonwealth v. Johnson, 420 Mass, at 467 (citations omitted). Cf. Commonwealth v. Vardinski, 438 Mass. 444, 451-452 (2003). By her failure to present evidence to the jury of the significant differences between the defendant’s appearance on the date of the shooting and the appearance of the person depicted in the photograph selected by the lone eyewitness to testify at trial, the defendant’s attack on the reliability of the witness’s identification testimony was substantially weakened. Moreover, counsel’s failure prevented the jury from hearing from the other victim (who — unlike Akers — had a prior acquaintance with the defendant) that the shooter did not have long braided hair. We conclude that the motion judge should not have denied the defendant’s new trial motion.13 “In light of the contested nature of this issue and the unavailability of the trial judge to hear a motion for a new trial, affidavits may not be sufficient to resolve the defendant’s claim of ineffective assistance of counsel. See Commonwealth v. Brookins, 416 Mass. 97, 104 (1993). Such a motion would necessarily involve fact finding that is probably best done through an evidentiary hearing. See ibid” Commonwealth v. Ramos, 66 Mass. App. Ct. 548, 553 (2006).
We need address only briefly the defendant’s remaining claims of error. Contrary to the defendant’s contention, there was no error in the admission of the prior written statements of the alibi witnesses, as the statements were offered for impeachment of the witnesses’ trial testimony, rather than for their truth.14 There likewise was no error in the jury instruction regarding the element of *525intent on the charge of assault and battery by means of a dangerous weapon; the instruction correctly informed the jury that the defendant must have intended to touch the victims, using a firearm to accomplish the touching. The further explanation (in which the judge stated that the defendant must have intended to “shoot” the victims) collapsed into one expression, perhaps somewhat imprecisely, the requirements of (i) intent to touch and (ii) touching accomplished by means of a firearm. It did not, however, as the defendant argues, incorrectly state that the defendant must merely have intended the action which resulted in the touching. Compare Commonwealth v. Ford, 424 Mass. 709,710-712 (1997); Commonwealth v. Moore, 36 Mass. App. Ct. 455,458-459 (1994). Finally, with regard to the charge of carrying a firearm without a license, there was no error in denying the motion for a required finding, which was based on the lack of evidence of the gun barrel length. In light of the circumstances of the shooting described by the victims, including the close proximity of the two vehicles and the absence of any statement by either victim of having seen a barrel, the jury could reasonably infer that the shooter fired a handgun rather than a shotgun or rifle. Cf. Commonwealth v. Sperrazza, 372 Mass. 667, 670 (1977); Commonwealth v. Manning, 44 Mass. App. Ct. 695, 707 (1998).15
The order denying the defendant’s motion for a new trial is vacated, and the case is remanded to the Superior Court for an evidentiary hearing.
So ordered.