I'm developing a 7 1/5" AR pistol in 300 Blackout with a suppressor. Coated cast bullets are costing me $30/250. Since you're shooting subsonic you'll want heavy bullets to maximize energy given recoil is never an issue in the AR. Save the jacketed 110's for your super loads.

I don't have a shoulder brace. Resting the buffer tube against my body steadies the pistol better than holding it off hand. A shoulder stock would be better. Pistol buffer tubes are different than rifle's.

I may be cheaper than you - paid $360 for my new complete AR pistol off of gunbroker.

Developing this pistol has been the greatest reloading challenge I've ever encountered. Balancing AR function with sub sonic loads while striving for minimal sound signature is a whole new world for me.