"drum" ... you mean "cylinder" :-)
The firing pin *should* strike in the center of the cap each time. On my revolver there was a problem with the cylinder where the cap recesses weren't quite machined deep enough for the caps I was using. This caused a couple of the caps to sit slightly proud of the cylinder which was enough to cause binding. Also the arm which the cylinder rides on kept working loose which caused a slight mis-alignment on the centre cylinder axis. Fixed that by tightening up the arbour retaining screw.
Primer pockets were fixed by deepening the primer recesses *very* slightly on an end-mill. The primers now sit flush and are being struck centrally every time.
Of course it could also have been that my primers (Remington 209's) are a little thick compared to others but since that's all I can get hands on locally, I had to go with them.
Anyway, it's working fine since the modification.
Regards
Paul M.
NSRA and NRA qualified RCO.