You got your answer in detail already, Roberteaux, but I'd like to share a couple thoughts on it, if I may... With an integral suppressor that meets minimum non-NFA barrel length, I believe only one tax stamp would be needed, as you wouldn't have an SBR, but only a suppressed rifle. With a removable (typical) suppressor on an AR pistol (so built with a brace like this instead of a stock) you could remove the suppressor when not needed and have a non-NFA firearm that could easily travel across state lines, be kept loaded and chambered in a vehicle in most states, and whatever other local laws may benefit the ownership of a pistol over a rifle or SBR. So you have an option to save $200 on a stamp and an option to have a "more free" firearm that also saves stamp money, if any of those may be concerns. I recently learned of another type of suppressor that has one of the acoustic chambers go back over and around the barrel, so the barrel basically threads into the center of it, rather than at the end of it, reducing OAL. For the life of me, I can't recall or find the term now. It's not "inverse suppressor" but something like that. Perhaps Mr. 45 or another here will correct me on this. If anyone has experience with one, I'd be interested to hear if there are drawbacks. I like the idea of a shorter OAL and a more-rearward center of gravity and will be either suppressing my 300BK or building another one with a suppressor, likely in this new year.