Just like a suppressor, when you buy a rifle with an 11.5″ barrel, it goes into NFA jail and you need to wait for your tax stamp to com in to pick it up.
AR pistols are not SBRs, by law, because they do not have a stock. ATF’s definition of a stock is pretty specific, which is why you can get away with arm braces, cheek welds, and the like.
First time NFA buyer here. The paperwork wasn’t nearly as bad or confusing as I was expecting. Hardest part was finding time to get fingerprinted. Anywho, I dropped it all in the mail yesterday and paid extra to get it to OR overnight. Now the fun begins, but I can’t wait to get my suppressor!