The Defense Distributed project tested a AR-15 lower they printed with a upper receiver chambered for 5.7x28mm FN. The 5.7mm cartridge generates a lot less recoil than a full power 5.56mm / .223 round.
Surprisingly the buffer tube threading held up, but the off axis force of the buffer tube against the ring caused it to fail. There is a write up about the incident at the Wiki Weapon blog.
[ Many thanks to everyone who sent in this tip. ]
[Hat Tip: Boing Boing]