SBR (Short-Barreled Rifle)
A rifle with a barrel under 16 inches or overall length under 26 inches — an NFA-regulated item requiring a tax stamp.
A short-barreled rifle is a shoulder-fired rifle with a barrel shorter than 16 inches, or an overall length under 26 inches. Because the NFA regulates these, building or owning one legally requires registering it with the ATF and paying the $200 tax stamp before you assemble it. SBRs are popular for their compact handling, especially with a suppressor.
The distinction matters because putting a stock on a pistol-length firearm, or a short barrel on a rifle, can inadvertently create an unregistered SBR — a serious federal felony. Pistol braces have made this area especially confusing and litigated. If you want a compact rifle, do it through the proper NFA registration.