Both the Full Auto and Semi-Auto Thompsons are pretty robust and I've noticed mine makes a larger impression on the primer. This may also be caused by the fact that they are "blow-back" and not locked breach. This means that the spring power and the weight of the bolt keep the chamber closed. I suppose when the round is fired there is a very very small amount of pushback that can occur that against the breach face and firing pin that would make the firing pin hit look larger than it is. Unless the primer is pierced I would not worry about it.