Maybe use one of these until Dead Air has what you want in stock.
https://www.brownells.com/rifle-parts/muzzle-devices/thread-adapters/thread-adapter-1-2-28-to-5-8-24-prod54364.aspx
Unsure on the round count. I bought it used because I wanted the LAR upper receiver for another build. I’ll have to check the barrel for the twist rate. I’m not home right now, but I will be later today.
I have these parts. It's a 16" barrel that I took off an LAR upper. It has a 7" quad rail handguard, barrel nut, clamp on picatinny gas block, gas tube, and birdcage type flash hider. I'd take $100 for all of it.