I need to know, if I have an AR pistol now.

When I get a NFA trust setup what do I need to do to use the short upper on a rifle lower to make an SBR or can I even do it legally then?

Thanks.

Whatever lower receiver you want to put the upper on needs to be registered on a Form 1 as an SBR. You can do that with the pistol if you'd like. Or, whatever rifle or stripped lower you want.

-Fill out the Form 1 with the information of the lower you want to turn into an SBR.
-Wait for tax stamp to arrive.
-Build your SBR.

Lets see if I have this right...

I can have a pistol with a shorty upper and I'm good (since its a pistol marked lower.)

If, later, I want to transfer that upper it to a rifle lower, I just do the paper and the pay and wait.

When the OK comes back I can permanently transfer the (prior pistol upper) to the newly dedicated rifle lower, correct?

I just want ot make sure I get this right.

Thanks again.

It doesn't need to be marked "Pistol". A pistol is a pistol due to it's configuration.

Yeah, if you want to turn a rilfe into an SBR you just need to do the form, pay, engrave, and wait.

The upper is just a barrel in the eye of the law. it's just parts. You can configure those parts however you want to, provided the receiver is legally capable of being configured in that way.

Got it! Thanks again!

ETA: One more question then.

I was going to by a multi-cal. marked pistol lower so I can buy a short upper, build as a pistol now and transfer the upper to  be a SBR later.

After, of course, the correct proceedures.

Or would it be pushing my luck to purchase a short upper now (they are on sale in many places) and store it off premises from where my AR's are w/o purchasing the pistol lower?

I don't want to get in trouble on a technicality but I'd really like to purchase a .300 BLK upper to use later.

I seriously am too old and crippled to screw with a law violation at this point in my life.

It shouldn't be a problem to have it in the same location as your AR15 rifles. The only way it could be is if someone actually checked, which is a one in ten million chance, and you had a lower with a rifle stock on it and no 16"+ upper for it. Then someone might infer that you mean it for the short barrel upper which would be a SBR when mated.

Thanks guys.

I was going to purchase a pistol lower so I would l be legal owning a short .300 upper.

I could always (hopefully) sale the pistol lower after I get a Trust completed.

I just didn't want to get myself in a jam.

No amount of "savings" would be worth that hassle.

Thanks again.

Where are you getting this "pistol" lower?  
An AR is classified as a pistol or a rifle depending on its configuration not on what's engraved on the lower. You can build a pistol on a new stripped lower that has never been configured as a rifle. Configuring it as a rifle means assembling the lower with a stock and mating an assembled upper receiver to it.

If you buy a lower receiver that has never been mated to an upper and it does not have a stock attached you can attach it to a short barreled upper and it is now a pistol. This lower receiver can now be configured as a pistol or a rifle.

A pistol can be changed to a rifle and back to a pistol but if it's built as a rifle first it is always a rifle.

The rules for short barreled rifles still apply. If you configure it as a short barreled rifle at any time you must first do the NFA dance and send in your form to get your stamp first.

Many guys build their gun as a pistol first so they can use it while waiting for the stamp to arrive. Then after they get their stamp they buy a stock and put it on completing their legal short barreled rifle.

I based this on AIM Surplus having a lower made by Anderson Manufacturing marked AM15 Multi. Cal. and AM15 PISTOL Multi. Cal.

If the manufacturers make a pistol marked lower then I figure it would be a good idea to have one on any upper with a barrel length less than 16" with, of course, the pistol configuration lower.

That's where I got the idea there were lowers specifically made to be used as AR pistols.