I own and shoot several: AR-10, AR-15 w/ silencer and 1928 Thompson .45cal. Nothing *impractical* about 3-round burst or full-auto, IMO. Please explain how impracticality is related to scarcity.
And to cover the sniper rifle angle, I have a Barrett M82A1 .50cal BMG on order, and am breathlessly awaiting delivery in October. Talk about impractical...
I wouldn't care to have my name and address on a BTF "full auto" record. Nore would I care to have an ATF agent drop by to check on things. I can do all the damage I need with what I've got now.