Old shotgun ammo
The older shotguns, like your Ithica were designed for 2 9/16 or 2 3/4 inch shotgun shells. The 3 inch and especially the 3 1/2 inch shells should not be used in them.
The reason is this. Basically, the shotgun shell is a cylinder of a certain length. This rating of 2 3/4 inch actually is a bit longer, as the crimp of the shotshell must expand into the chamber when it is fired, so to make room for this, a 2 3/4 inch chamber is actually closer to 3 1/8 inch long.
However, the actual bore size of the shotgun is a lot smaller than the diameter of the shotgun shell, somewhere in the neighbourhood of the INSIDE diameter of the shell. The chamber of the shotgun is bored for the OUTSIDE diameter of the shotgun shell, to allow it to unfold when fired.
A short CONE much like a shallow funnel top connects the front end of the chamber with the actual bore size, and directs the shot charge into the barrel. With a proper length shell, this system works as designed.
When a longer shell is loaded into the chamber and fired, there is NO ROOM for the crimp on the end of the shotshell to expand and lie flat. It is then forced to expand INTO THE FORCING CONE. This creates a restriction, and the shot charge then has to compress more due to the smalled diameter at the cone. This restriction raises the pressures greatly in the barrel.
When you add to this (and we might as well call it what it is) bore obstruction, the fact that the longer 3 inch and 3 1/2 inch shells are generally loaded a bit higher because they are game loads, the pressure rise is much more than the gun was designed for.
