Well, I may have hit the happy spot

Not that, you bastards.

Subsonic test today
DISCLAIMER:
There is no published handloading data for this bullet and cartridge, so I'm going by estimates, what some people have written about, experimente, and guesswork.  This- so far- has worked for me, which does not mean it'll work for you in your rifle.  If you give this a try, it's all on you.
7.62x39, the Hornady 255-grain Sub-X bullet, 11.7 grains(0.2 grain increase), CCI LR magnum primers, in Winchester cases, fired over the Chrony: average 1028fps.  That's five shots to the low & right, the other two were something else.  25 yards

I think from a better rest- with less cold air on the back of my neck- I'd have had one ragged hole.  Primers look like normal after firing, no high-pressure indicators I can see.  I think this'll be the one to test more on when I can get to a longer range.

The previous test was average 977, and I wanted to get it to 1000 or just over, since these bullets are supposed to reliably expand 'down to 900' and I wanted a bit of a cushion while still staying well subsonic.  If this continues this way, I've got the load.

I  know that, realistically, 100 yards would be the max for expansion and good accuracy, maybe 150 if this tests out on that extra 50.  It's really nice when something is working out.