Okay, 12-rds might work if you're running a 9mm. Seriously, if you read the OP's update, his gun appears to have all new internals, including the barrel. Shooting 300-rds of real 10mm ammo not only lets the OP vet Glock's warranty work on the gun, but those 300-rds also help "break-in" any new components Glock installed. As well, the extra range time needed to fire 300-rds will definitely determine whether POA/POI discrepancies are shooter errors, ... or a gun issue. It sounds like the OP's original issue is fixed, but firing a few hundred rounds of balls-to-the-wall 10mm loads will verify it - one way or the other. He can thank me later.