End goal: I want to build a 16.5" OAL AR Carbine, utilizing a pistol length barrel of approximately 10.5" and a permanently attached Form1 can to bring OAL to 16.5" Intended uses: Primary 'Bump in the night' weapon for two or four legged predators, inside or outside my rural home. Recreational shooting without hearing protection. Throw it on the quad weapon when working on the property and threats or recreation arise. Possibly use for deer hunting. Quandary: Go 7.62x39 or .300 AAC Blackout. 7.62x39: I've built and own a 7.62x39 AR, and it took quite a bit of tuning to get it to run right. Subsonic loads are expensive if available over the counter. Subsonic load data for 7.62x39 is sketchy, brass is expensive, and I don't get the warm fuzzies that I'll be able to develop a load to run 100% through an AR. Special mags needed. Pluses: Supersonic ammo cheap and plentiful. Pistol length uppers cheap through Model1. .300 AAC Blackout: Factory subsonic loads readily available. Subsonic data readily available. Brass readily available and cheap. Parent brass readily available if you want to roll your own. Factory mag usage with no mods. Barrels reasonably priced if I retrofit a 5.56 upper. Cons: I haven't rebarreled an upper before, so there's a learning curve and a parts and tool investment. Lack of cheap, bulk, loaded ammo. Newer cartridge with short track record. Any input, thoughts or experience on such things are wanted and appreciated.