I think NGB said it was 'theoretically possible' that a second rifle was used, but in his view highly unlikely. Some of the recovered bullets were in a state where they could be positively identified as having been fired from the Anschutz. Others were so deformed that it was not possible to say whether they had been fired from the Anschutz or not. That therefore leaves open the theoretical possibility that another .22 rifle was used.
If the Brno rifle was at WHF that night, with its silencer, it still couldn't have been fired without its bolt and even Jeremy agrees that the bolt had been removed. This is consistent with Pargeter's evidence at trial, although in his statement to COLP, I believe he said he took the whole rifle from WHF a week or so before the tragedy? However, I think they can both use the same ammunition so the possibility remains.
If I remember rightly, NGB said that Andrew Hunter's claim that a .223 Ruger carbine was carried by one of the raid team, but it's use as a murder weapon could be ruled out because this was a high powered centrefire rifle and at close range, a shot to the head with the Ruger - would have blown the head apart.