The ATF just approved an AR-15 transfer for me. It is an old SP-1 legally converted before '86 and is fully transferable.
That said, who the heck can afford to feed it? I have seen Lakeside Machine's LM-7 belt fed .22LR upper. The owner once worked for Tippman when they made the 1919A4 mini clone in .22lr. I am a bit excited by the prospect of 100 rd belts of ammo that costs under $20.
I have heard mixed reports about how well they function. Some guys say they jam, others say they run well if you use the right recoil buffer. Others say they are ammo sensitive.
Does anyone in the forum actually know?