What is the shortest barreled upper you can mount on a registered M16 before it becomes a short barreled rifle? Thanks!

i don't think it matters, as it's a machine gun, but i could be wrong.(just ask donr)

On a registered machinegun the SBR rules dont apply. So the answer to your question is NEVER!

So you are telling me that the Dept. Of Energy pistol upper on ebay last year that sold for $700.00 could have been legally mounted on my lower receiver and made into a pistol?

Or are you saying I am the biggest idiot on AR15.com?

Or all of the above? [thinking]

Ok if you have an NFA registered machine gun reciever there are NO restrictions on the barrel length nor are there any on the weapons OVERALL length. Now converting it into a pistol I dont know of any laws restricting that but wait around here for a bit and im sure some of the C3 guys will have an answer for that.

osprey whats your sig line all about?[?]

The DOE upper could legally be mounted on a registered full auto M16 lower, or on a lower with a registered machinegun conversion device (lightning link or DIAS) installed.

It could also be legally mounted on a pre-ban lower receiver which has been registered as an SBR ($200 tax, law enforcement signoff, pictures and fingerprints, long wait).

It could be modified (removal of the flash suppressor and bayonet lug, if present) to mount legally on a post-ban lower receiver which has been registered as an SBR.

It could be legally mounted on a pre-ban AR-15 pistol receiver without modification.

It could be legally mounted on a post-ban AR-15 pistol, provided that the handguards and any other evil features (flash suppressor, bayonet lug, if present), and provided that the assembled pistol is either under 50oz in weight, or the magazine has been made non-detachable.