Just found this thread, and would like to throw in a couple of cents worth, as I’ve been working up loads for my newly converted 300 Blackout AR…
Quick caveat… these are results and opinions based on my rifle and my loads… YMMV…
First, people talk about this load or that load operating the action in an AR chambered in Whisper vs. Blackout vs. Fireball, etc… I’ve rarely heard people talk about which barrel they’re using, and the size of the vent under the gas block. My barrel is a 16 inch CMMG purchased from Midway, and the gas vent is significantly larger than the vent on my .223 barrel. I’m confident that the extra gas enables my rifle to function normally with loads that other shooters may have trouble with if they have a standard .223 vent on a 300 Blackout barrel.
That said, my last trip to the range included some cast boolit loads using 5744. It was cold and a little windy that day, so I only shot at 25 yards and checked mainly for function. A couple loads were too light to cycle the action, and others were just fine… Using a Lee 200 grain boolit with a gas check, I tried to make it sub-sonic with 10.7 grains of 5744… I’m sure it was sub-sonic, but the action would not cycle reliably. Short stroking; most ejected OK, but the bolt never stripped off the next round in the magazine.
Dropping down in weight to the Lee C123 and a GC, I used 17.5 grains of 5744. This load cycled very smoothly. The primers showed some slight pressure signs. Moving boldly forward, I also loaded boolits from my 311329, about 190 grains with GC, over 17.0 grains of 5744. That was a snappy load! No function problems here, but the primers definitely fill out the pocket. I’ve shot higher pressure loads, but I see no reason to load them this high… I’ll back off in the future.
As far as the loads referenced earlier, the current thinking on other forums is that AA1680 is a good choice for the heavy bullet, sub-sonic loads. H110 seems to be the favorite for small bullets at super-sonic velocities. I’ve used it with small bullets, and it works fine but I don’t think I’ll use it for cast boolit loads.
Currently, Swede at NOE (on this forum) has a 247 grain boolit designed for 300 Whisper scheduled to build next October. Tom at Accurate Molds (
www.accuratemolds.com) has a few heavy designs for Whisper/Blackout also. I have one on order, and am anxious to give it a try in my rifle.