It would be "Extremely" doubtful with that many bullets stuck in the barrel, especially as you did not mention a bulged barrel, that any of them reached maximum breech pressure. Sounds as if he had some bad ammo of some sort, which did not reach maximum acceleration.
I do not believe that an un-bulged barrel is a guarantee there was absolutely no check to the projectile, but simply there was not enough check to produce a bulge. The severity of the hammer would of course be dependent upon the extent of the check. It would seem unreal that if the ball is large enough to be engraved by the rifling that at least a "Slowing of Acceleration" if not a slight check would indeed occur.
