Some of the older mossberg shotguns were pretty good!!! My dad has a .410 pump thats a mossberg and its a good little gun... i watched a buddy struggle with his Mossberg/Maverick 88 with three inch shells for a couple years, his was a worn out "keeper" on the magazine that holds the next round from popping out before you pump it... Little file work and it was good for a while but then it just wore down again and started jamming again... don't know that this will help you any... The other thing to check is to see what the actual length of the shells are... Most 3" are 2 3/4" before firing and 3" after firing, but I have had some Kent 3 1/2" shells that wouldn't eject properly because they were "too long" after firing, depending on both the gun manufacturer (I am assuming the gun is chamber for 3") and shell manufactures tolerances this could potentially be the problem... You could try a different brand of Shells and see if that helps you or not... Does it handle 2 3/4" shells with no problems???