As long as the upper is only ever pinned to the rifle lower when the suppressor is applied to the upper first, no special lower is required correct? Else a pistol lower can be used with the SBR upper with no paperwork required.

Is the suppressor permanently attached?

X2

Wrong , the can has to be pined/welded to barrel to make it legal

this, it must be permanently attached and be 16" long to be a one stamp gun, or run it on a pistol lower but it will still require a stamp for the suppressor

Nope, still two stamps.

UNLESS you buy a integral suppressor and the overall length of the barrel and suppressor is 16" (Suppressor can not come off, its either blind pinned or welded).

For the record, most people recommend NOT welding on a can.

I hate it when a plan falls apart.

2 stamps no matter if you weld it, stake it, super glue it or use gum to attach to the barrel..... you are not going to be able to procure the suppressor without a stamp or take possession of the lower without a stamp

You can own an sbr upper if you own a pistol lower, when welding a suppressor to the sbr upper it SHOULD make it a standard 16" upper.

Now if you don't have a registered lower or pistol lower you are taking a risk at having an illegal rifle in the house.

The only tax stamp you will have to pay is the suppressor tax stamp, if it was me I would go ahead and pay the extra stamp because now you are looking at paying around the same to get the all the gun smithing done for something small plus you can take the suppressor off and clean it regularly.

I'd rather do two stamp taxes so i could have a can that goes on multiple weapons.

That would be the best way to go, some people want to save the extra 200 bucks on an SBR stamp. Scalpel arms was making 1 stamp .300BLK uppers not too long ago.

The lower doesn't need a stamp if the suppressor is permanently attached to the barrel and the combined length is 16" or more. So it's not necessarily a two stamp gun.

I had a 11.5" upper that I welded a Tac-16 suppressor to. Brought it out to well over 16" so no stamp needed. I'm planning on attaching it to another short upper, welding it on, then running a rail out over the can. I never use that suppressor anyway, so I might as well do something with it and a 16" barrel with a suppressor is just too damn long.