Since a super quality trigger isn't essential for a shotgun, and Benelli's trigger on the M4 is pretty good, I'm thinking about just paying the $200 tax stamp and registering it for the ability to use a short barreled shotgun. I don't have a short barrel for the M4, and I don't plan to get one, but that $200 tax stamp would exempt me from the 922r nonsense. It would be like "just in case" insurance if you wanted to upgrade a few pieces of the gun and not have to buy a handguard, trigger group, or hammer that I don't want/need.
And if I ever got the hankering for a shorty M4, I just have it shipped to me and slap it on.