18?1 is what the factory barrel nut and birdcage flash hider threads ont. If you have a smaller diameter threaded section ahead of those threads than you have 1/2?28. I'd give you a picture but i currently have a comp redloctited on
As for comp suggestions the SLR synergy minicomp I'm running is pretty nice. Also have fortis and nerd making higher end comps aswell. Most other stuff around is just 5.56 designs drilled out to pass 9mm projectiles.