The problem with the glock lowers is that the ejector has to be removed and the mag height has to be retained at the correct height. I use the cmmg rdb with a standard ar lower and endo mags. I really like that setup.
I wonder how short barreled shotguns would play into this ruling. They are different classifications per the NFA but not mentioned from what Inhave read so far.
From what I have seen of the design, if you were to pull the trigger back really hard it will bind up the bolt carrier or break the trigger. If you can get off the trigger before the bolt closes then it would only fire a single shot as well. Same goes for a FA as well though.