Help me understand... building a ar 15 .300 blkout pistol

[MacFromOK]

ha thank you, i was just reading that before you posted it... i think it clears it up. the answer is yes you can, as long as you do not modify it and it is indeed a stabilizing brace/ arm brace..

I suggest reading the comments, if you haven't done so.

 

[cktad]

Short answer: no.

Long answer: Only if the lower is registered as an SBR *prior* to installing the 10.5” upper.

Maybe, maybe not as the BATF clarified the use of the pistol brace and said shouldering it is not a redesign so no SBR. Also can't modify a pistol brace to make it easier or more comfortable to shoulder.
https://freebeacon.com/issues/atf-reverses-controversial-stance-shouldering-ar-15-pistols/