9" to 10.5" is the optimal barrel length for .300blk, i think. I'd pick a longer hand-guard with a big i.d. that a suppressor would fit inside, then it could handle like a rifle. Some sig MCX's come like this.

The Brownells BRN-180 and their retro BRN-180 lower are on my bucket list

If I had $ to burn I'd send the upper, handguard, lower out for a sick themed cerakote job before assembly.

Buy a few dozen .300blk specific mags, preferably in a different color than your 5.56 mags. Don't mix them up, since mags should always be loaded...

9" to 10.5" is the optimal barrel length for .300blk, i think. I'd pick a longer hand-guard with a big i.d. that a suppressor would fit inside, then it could handle like a rifle. Some sig MCX's come like this.

The Brownells BRN-180 and their retro BRN-180 lower are on my bucket list

If I had $ to burn I'd send the upper, handguard, lower out for a sick themed cerakote job before assembly.