16" upper on a registered SBR lower?

[Rbs]

I'm planning on getting a suppressor for my AR and 10/22... but since I'm going to wait for ATF paperwork I figured why not SBR my AR.

My question is if I register my current AR (16" S&W middy) as a SBR can I legally keep my 16" upper on it until I purchase/build the "short" upper?

 

[chopper duke]

If you do that, Ninjas will materialize out of your walls, kick your ass, drink all your milk and steal all the loose pennies from around your home. But it's not illegal.

Edit: real answer, it's fine. It's still an actual lower so it can be used as either a pistol lower or a rifle lower.

 

[heavyd]

Fill out the Form 1 with whatever length barrel you will put on it, be it 10.5" or whatever. You may absolutely keep your 16" barrel on it and any time after you receive the tax stamp.