I understand that on some semi-auto shotguns, shortening the barrel can affect reliability with light loads. Is there a way to correct this? The gun in question would be a Winchester Super X3/FN SLP and the barrel would go from 22" to 18.5"
With that gas system you will be fine. Once the gas has energized the system, it will cycle unless you get really crazy with it like cutting the barrel off 1 inch past the gas port. Recoil operated is a different story though.
Avoid mossberg semiauto shotguns for anything more than just range fun. The most reliable one ive had experience with still required a detailed cleaning after 4 boxes of shells. Most of them are single shots based off the 4 Ive messed with.
I just did this. Used my metal chop saw and stayed ahead of gas port. Had to hand file about 1/64” (debuting more than anything) to get vent rib absolutely flush and had no issues.