A suppressor with no moving parts is considered a firearm by the ATF soooooo yeah.

18 U.S.C. § 921 - U.S. Code

(3)  The term “firearm” means (A) any weapon (including a starter gun) which will or is designed to or may readily be converted to expel a projectile by the action of an explosive; ?(B) the frame or receiver of any such weapon; ?(C) any firearm muffler or firearm silencer; ?or (D) any destructive device.  Such term does not include an antique firearm.

Regarding the OP's intended non-firearm project:

Whatever paint's one's wagon; I sure hope he posts photos and a video of the completed air rifle in action.
There are plenty of true full auto airsoft and air rifles out there, so I'm not seeing any issue.
I read of a related project attempted during the Civil War, using steam power, but it was too cumbersome to be of practical use.
Decades back, someone advertised and sold a full auto BB gun, powered by R12 freon cans. EPA heart attack today.  I still have one, and it worked.

In specific response to the title of the post, and very concerning:
The Dec. 2018 Issue #23 of Firearms News shows ATF letters declaring a bolt action .50 Cal AR upper to (now) be a firearm, requiring a serial number.  
This is the upper only, with no lower involved.
If the upper is attached to a lower, the assembly will now have two required serial numbers.

W?
T?
F?

"Any destructive device"

.22 powered nail shooters are constructive, there you go. The device OP wants to build is going to tear down something ie "deconstruct" it thru propelling projectiles.

I don't see the ATF letting you get around that one. They want their Stamp fee. If you move fast you can buy a bumpstock and enjoy it before it sunsets in the next 48 hours.

Full Auto small ball bearings

There are full auto bb guns that are replicas of real guns.  They are legal to buy over the internet with no ffl or 4473 or anything. You can buy a suppressed muzzleloader shipped to your door. They make big bore air rifles. I don’t know why you couldn’t make a full auto big bore air rifle. I had a couple paintball guns that were select fire electronically 17 years ago.

I would plan on something like a 3/8” or 1/2” max bore to make sure that you are not going to fall under destructive device gray areas.  It would be nice to be able to use off the shelf projectiles. Ball bearing or swaged round lead balls would be the easiest, but it would be cooler to use a .38, .40, .44 pistol bullet. It wouldn’t be hard to make a magazine for .38 wadcutters due to the shape.

Instead of going the electronic war, you could go old school mechanical gatling gun and even go as far as motorizing it into a mini gun.

Air flow will be the biggest problem. You are going to want high pressure with high flow rates. Maybe a scuba tank running through 3-4 paintball regulators set at 600 psi would be enough to have the power with the sustained full auto. Flow rate through the regulators will be hard. You will definitely outrun a single paintball regulator. It will start out shooting a nice flat trajectory then fastly go to a rainbow arc. On my $2000 (yes I had more money than brains back then) semi auto paintball gun I had to replace the regulator to keep the fps even at 13 rounds per second. On the full auto gun it started to drop off at 15/second. At 20/second it was only usable in bursts.

You’re going to use it lot more air getting a heavy projectile up to speed than you will getting a.68 paintball up to 300fps. I would shoot for 6-800 fps as a goal. Less than 600 would suck and more than 800 would take a ton of airflow without having to slow down the cylic rate. .38 lead round balls at 600fps and 600-900 rpm is doable on a scuba tank. Even a 114ci 4500psi paintball tank would run it for a couple hundred rounds. Most of us ran 68ci 4500psi tanks that we would refil off of scuba tanks. My team had 4 scuba tanks and by filling in order tank 1,2,3,4 we could maintain a good 2500 psi throughout a long tournament day.

Your quest of full auto destruction with air power is probably doable, but it’s right up there with my paintball tournament days as far as down the road regrets.

I would go but a full auto bb gun for a couple hundred dollars and have cheap fun destroying pop cans instead of cinder blocks. To do it right, you’re easily looking in the 4 digit price range and that is if you have the machinery to make it happen. If you have to have a machine shop make one off parts, it might even be closer to the $10k mark. There just aren’t many things out there that could handle the flow and pressure fast enough. The best way would be to reverse engineer an electronic paintball gun. Enlarge the valves and internals by 2-300% and reduce the barrel from.689 to <.50.

The interesting thing is that a firearm is federally defined as "designed to or may readily be converted to expel a projectile by the action of an explosive". Note an explosive, which has a specific legal meaning that may exclude propane. Remember that black powder and smokeless powder are both considered "explosives", but are specifically exempted from typical explosives regulations (FEL, magazines, etc.) so long as they are used in small arms ammunition.

So, a weapon that uses an explosion (but not an explosive) may not be legally considered a firearm. Operative word being "may". YMMV, IANAL.

What ever those in power define a firearm to be. Definitions change all the time.

Tippmann made the C3 Paintball gun. It is propane powered.





ATF never classified it as a gun. Yet it used combustion and the expansion of said combustion to propel the projectile.

But ATF did a while back deem that Daisy's VL rifle is a firearm since compressed air ignites the caseless ammo.

Fun fact, Tippmann used to make the mirco belt fed .22 MGs.

Potato cannons aren't DDs either

Personally I would avoid the DD classification by making sure to stay under .5" as the definition of DD doesn't need to be expelled by explosive:

"any type of weapon by whatever name known which will, or which may be readily converted to, expel a projectile by the action of an explosive or other propellant, the barrel or barrels of which have a bore of more than one-half inch in diameter"

The reason potato cannons aren't DDs is because they have been determined to not be a weapon

But be warned, if you classify it a DD then it is a weapon by definition.

And then it meets the definition of MG: "Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot without manual reloading, by a single function of the trigger"