There exists a very serious doubt that both bullets, PV/20 and PV/19 were fired from the same gun - otherwise, why swap over the original fragmented piece, for a whole control bullet, to enable the ballistic expert, Fletcher, to link both bullets to the anshulz rifle?
Either:-
(1) both bullets, PV/20 and PV/1, were fired from the use of the same gun, one with the silencer attached, and the other without it attached...
or:-
(1) one bullet / pellet, PV/20 was fired from another gun, withy the damaged silencer (SBJ/1) attached and PV/19 was fired from the anshulz rifle, minus the damaged silencer
However, whichever weapon was used, any bullet / pellet fired through the damaged silencer (SBJ/1) would have shattered it, in the same way that the original PV/20 appears to have been shattered, which serves to prove and establish that the fatal bullet, PV/20 could not have been, and was not fired through the damaged silencer at all...