Cool. What's your total length with barrel & suppressor?
Cool, so 9" is good. Your 8" still run good? Fireball? Total barrel & suppressor length?
5.5"?! What's the plan for it? Suppressed?
Cool. I'd like to keep it very similar to an 11.5" pistol.
Is it worth it to run subs in a non-suppressed 9" .300BLK?
My total length currently with the Rex Silentium MG7 is 9"(barrel)+
5.6"(can)=14.6"
With the Sandman K its 9"+5.4"=14.4"
With the Omega 9K its 9"+4.7"=13.7"
With the AAC 762 SDN-6 its 9"+7.6"=16.6"
I honestly think a 10" barrel is the sweet spot for powder burn with 300blk, but it also depends on the can. My Sandman-K is not designed for quiet, its made for fighting, so its the loudest and has the biggest flame. But they make a flash hider endcap for it that only adds like 2mm of length, but tames that fireball tremendously. My MG7 and Omega 9K do much better jobs because of their extra volume (think diameter) and baffles. And they also don't have swappable endcaps. The AAC tones that shit waaay down but its heavy and long, its old tech really.
300blk really shines using subs/suppressed out of an sbr, its pretty much what it was made for. It is not worth it to run it unsurpressed, but you may consider QD mounts (think muzzle brakes/flash hiders/hybrid comps) in case you get a baffle strike or something and have to send it in for repairs, so you can still use your weapon. I use muzzle brakes on all my cans except the Surefire RC2 which uses a flash hider. On the RC2, it gets a better gas seal with Surefires flash hider versus their Warcomps and brakes, but that is an exception to the rule. I use brakes because the brake baffles act as sacrificial baffles to prolong the life of the can.
If I had to do it all over again though I'd just get a SIG MCX, but I still love my little 300 its my favorite.