What constitutes a blank firing gun in the US.

State laws aside, what constitutes a blank firing gun in the eyes of the ATF?
I've looked but I couldn't find any information on it.

I've always wanted a full auto blank gun but they're so expensive.
I have the skill to make one, but I wouldn't want to run afoul of the law.

Thanks.

The ATF would have to determine both that it was not a firearm and that it is not "readily convertable" something that is of course a bit subjective. Unfortunately there's no handy database of criteria or even of models that have been cleared, and when something is approved the notice is by way of a letter to the person who sent in the sample, and that letter is considered a confidential correspondence. Not helpful! You can ask their agents or have something sent in for evaluation if it's about import (assuming no government shutdown is currently in effect for them, as it probably is right now), but since you'd have to make it first to send it to them, you'd be on dangerous ground there legally. If it was already determined to be a blank gun, non-firing and not readily convertable, then you could probably convert it to full auto without it being illegal... but I'm no lawyer.

Been doing some research about all this lately for a book that comes out in the Spring.

Visit Indianapolis Ordnance's site - they have approved their blank fire only submachineguns, they don't give a lot of info, as that was their hard work and they obviously want to keep that to themselves.
But, an off diameter receiver (meaning not exchangeable with the real thing), off diameter bolt, blank only barrel welded in place, firing only proprietary caliber SSRoom 9.3mm blank rounds and each new model is sent to the ATF for thumbs up.

They are an 07 FFL paying for a(n) SOT so they have no problem assembling post sample machineguns legally.

Without the manufacturers special operating tax, you should put together an engineers blueprint and submit that before building anything.
Or, buy one of the Turkish 9mmPA full autos, they are cheap, and work pretty well.
(I wouldn't buy one of the SSRoom guns - at least the IO guns can fire with the mag the correct way around.)

Good luck!

Thanks guys for the replies, I was afraid there wasn't a set definition but I figured I'd ask anyway.

Like Fight Designer said I am thinking of converting a blank gun to full auto since it's allready a blank gun, I will contact the ATF first since their rules are notoriously crazy, and punishments are severe.

We've used a few of the Turk Jackel's - they're great - I used one on so many productions, it eventually fell apart, lol.

Pit.