I was going to wait for the weekend; but it looks like our weather is going to take a turn for the worse so I decided to go test the 180 hotcore loads today. Conditions were light winds but in the mid 20's--pretty cold for shooting bare-handed.
I started off with groups that were still in the 2 MOA range--when I decided to check the weaver mount--and of course it was very loose; a lot of ammo was wasted before I thought to check that. Since I read of many recipes using CFE BLK--I know from experience where it does well--AA1680 usually does as well or even better. However, I did not get the velocities that quickload told me I might, so that was a bit disappointing.
The use of 358 hotcores sized to 357 was courtesy of mehavy on this thread--I probably wouldn't have thought of it on my own had he not suggested it. Turns out it carries a couple of bonuses: It not only shoots well but allows a long seating in the chamber--going right to the canallure gets you around 2.27 but still conforms very well to the chamber. Despite the modest velocity I got with AA1680, it still leaves some extra room to work with.
I honest-to-God called the fourth shot in this five shot group as it broke, I knew I had pulled it and blew it. Otherwise, a pretty nice group compared to the ones I've shot so far.
This cartridge remains an "oddball" to me in an AR. It's true the felt recoil is significantly less than say a 5.56--but I find the concussive report tremendous--by a factor of several times that of 5.56. By the time I left after shooting 25 shots I had a bad headache and my teeth hurt. It's possible the R Silentium brake may have something to do with that--it has radial ports all around the brake which are pointed back. Even my iPhone siting next to the gun which I used for timing the shots got "stunned" a few times.
My intuition is perhaps a significant amount of the powder's energy is getting wasted in muzzle blast--but I'm not sure about that. Interesting how I've read that other's find the muzzle blast very muted. Nothing I've shot including factory ammo so far is like that.