How do you mean? Is the shell too long when fired? Is the plastic down the barrel? Maybe snap a photo or two. It may help us to visualize (plus there's at least one actual gunsmith here)
Most likely the hull was slightly too long and the end opened up in the forcing cone. I would use a piece of wood doll rod and try and smash the crimp back down into the cylinder. Don't use metal and scratch the barrel.
IF memory serves me right on this---I wonder...Why is a shotgun shell after being fired called a hull?...![]()