The general rule of thumb is 10.5” for 5.56. 8” for 300k. I’ve been told this by a professional class 3 builder. This is also the general consensus on the AR15 forum. .
I built a PSA 8.5” 300 with rail, flash can, Sig Romeo 5 and adjustable SB3. Also a 7.5” 300 with shockwave brace, A2 and Troy...