barrel length

[Marksman]

Ok, Barrel length on said AR is 14.5" with a 1.5" muzzle break making the min 16". Is it lawful to put an adjustable stock on this rifle or would that make it an SBR?

[JayBell]

barrel length for NON NFA rifles is 16" MINUS any non permanently attached muzzle devices.

this is from closed boltface to the tip of the barrel. for a 14.5" barrel to be legal on an AR, you need to permanently attach the 1.5" long muzzle device to it, via one of the approved ATF methods:

high temp silver solder
full circumference welding
4 equidistant welds
blind pinning.

as for OAL, or "Over All length".. the minimum is 26". this is measured with the stock open, so provided you meet the above barrel definition, yes, a collapsing stock may be installed, as the open stock gives a OAL well over 26".