You can always build an SBR (short barrel rifle), it's an extra $200 tax stamp, paper work and waiting for the ATF to file everything though.
other than that if you aren't going to SBR it, you just buy a 16" upper, most of the time it isn't economical to build an AR upper, I would just buy a conplete upper assembly.
Correct me if I'm wrong here. As long as the overall length of the rifle is >=26" (with the stock fully extended), it's not required to be classified as an SBR. If it's shorter than 26", I would need to fill out a form and pay a $200 fee. It doesn't matter what the actual barrel length is.
I see. So the barrel has to be at least 16" and the overall length must be at least 26". Got it.
What about a supressor? Is the supressor considered to be part of the barrel?