I use Remington nitro mags in 2 3/4 with a 1.5 oz load. I have a few shot guns one of which doesn't take mags or anything larger then 2.75. Can an average gun handle this load or calling the shell nitro mag a gimmick?
You should be safe with nitro magnum loads with any guns provided they don't have Damascus barrels. In reality, most modern shotguns (1960's-present) should be safe even with 3" magnum shells if you had the chamber altered. Of course, the disclaimer is that they probably shouldn't be shot heavily with these rounds. I've seen many shotguns come into my dad's shop that were in very rough shape from shooting too many heavy loads. Mossberg shotguns seems to wear quite a bit with repeated shots with heavy loads. Under normal hunting use, there shouldn't be any problems.