I have a M1S barrel/upper that I'm very pleased w/ that shoots sub 3/4" groups w/numerous loads both super & sub.
Paul @ M1S is very glad to help you if any issues do arise.
However I do not know his response to 12" barrels.
Mine is 16" & ported @ the carbine position & works great w/subs & supers & no adj valve.
Call him & ask his opinion if you like. 903-546-2087 or email him @ (I think)
sales@model1sales.com