Make sure that the threads on the barrel are concentric to the bore before you shoot it with the can on.
Take the gun apart. Put the can on. Look down through the barrel from the receiver end. You should be able to see if the can is lopsided or not. The human eye is pretty good about recognizing if something is centered or off center.
You are going to need to clean the gun more often because of the blow back from the can.
Stopping power: The power some guns have to make you stop what you are doing and get your wallet out.