If I use an ar15 rifle lower, can I use a 10.5" barrel plus a 6" suppressor (making it 16.5") and it be considered a legal rifle?

So would I need to pay the tax stamp initially for an sbr or is there a way to have it pinned and welded and not have to?

By the absolute letter of the law, if you connect the upper to a lower without the pin/weld, you need the stamp. If you never connect the upper until after the muzzle device is pinned/welded, you're fine. Let your conscious be your guide.

Though honestly, I'd just get the stamp. It's fairly quick and you'll want to be able to remove your suppressor for cleaning or maintainence.
 
Pin and weld it before putting it onto your lower and it is a one stamp (suppressor) gun.

If you don’t P&W it, it is a two stamp (SBR and suppressor) gun.
 
Also, by the letter of the law, you could get dinged if you're in possession of the SBR upper and don't have a legal lower for it. Meaning a pistol lower or a stamped sbr lower. You start getting into "constructive possession" and such. The easy fix for that is to have a lower sitting around with a pistol tube on it.

But this would be a ridiculously rare scenario. To be caught and charged this way.
 
The best way ...if the SBR and Suppressor are legal in Your state...is register a short barreled rifle...then register the suppressor...that way a suppressor can move to other 16" barreled guns too...or other already registered SBRs of same or smaller caliber in your collection...instead of a permanent fixture on one SBR, I find that a terrible idea, for many reasons.