A buddy of mine has one of these that he uses with his Class 3 Full Auto lower (.22LR is a LOT cheaper than .223 when you're shooting FA).
It works well for a while but it tends to foul quickly. Definitely need to run good ammo. Basic lead ammo fouls after just a couple of magazines.
Jesus said, "I have come to cast fire upon the earth; and how I wish it were already kindled!" - Luke 12:49