Look at it this way...
If it has a stock AND is less than 26" in TOTAL length = SBR, needs stamp.
if it has a stock AND a barrel less than 16" in length = SBR, needs stamp.
if it has a stock, it is a rifle...
if it is a rifle with a short barrel or short overall length, it's NFA stuff.
Many SBR's have a short barrel and a short OAL.