The fix, of course, is to only use USA made ammo. Some ammo is made on the loose side of the tolerances, some not. Every once in a while I'll run across a shotgun that FTF's with just one brand of ammo,and not with others.
One thing, how old is that 500? Some new stuff must be broken in, and some older shotguns have crud buildup in odd places. Try a good cleaning and lube job, see if it helps....