One problem that can arise with rifle caliber inserts in shotguns is primer flow into the larger firing pin holes in the breech, which can lock up the gun. It can be a problem in older riles also that were built for black powder ammo. The problem is worse in guns with rebounding hammers as there is nothing to hold the firing pins forward. I have a full length insert in caliber .40-50 Sharps bottle neck that may have been made by Remington that I have never tried because I don't want to fool with black powder cleaning.
