With mine none, It will shoot anything. I have done no mods,there are correct ports, built in 06. At fisrt it didnt shoot squat consistantly, AT ALL.
3 out of 5 would hang up using low fps bulk packs. After reading and searching around this site I learned to feed it some higher FPS for awhile, ya it cost more but it will break itself in too a 100% reliable shotgun. I use proper setting for all loads
Mine will now eat every single round i feed it, it spits it all proper. low bulk to high.
My two sents as well as others feed it 3"00buck and anything else with high FPS
its nice to