Can you transfer a M4 AR15 16â€™â€™ to a 11.5â€™â€™ SBR? Or do u buy a lower than the upper and fill out form 1 or with ever form that does not have a trust. I can buy a 11.5â€™â€™ SBP upper for around four hundred and a complete lower for two fifty. Or buy a SBR from The AR Bunker for around thirteen hundred plus two hundred NFA stamp. If I bought a SBP and put the upper on my M4 lower that would be illegal. What is the easiest way to get or build a SBR?