Went ahead and ran 50 cases through the rifle. Used Lee 55gr .224" bullets with a gas check, powder coated, then sized to .224". Loaded over 8.5 grains of AA#2, seated out just long enough to get a slight jam fit into the lands so that the brass would be consistently held back against the bolt face when fired. Everything went as expected. Accuracy was bad, but that was kinda beside the point. No real fouling in the bore. Cases didn't show signs of pressure except perhaps very slightly flattened primers on some cases. These are WSR primers, so notoriously soft cups. Most importantly, nothing blew up and I had no misfires or hangfires as I worried with such low case fill.
I took the cases home, deprimed without any sizing using the Lee universal decapping die, and measured base to shoulder datum using the Hornady comparator and the A330 bushing. I had measured a random sample of 10 cases before I loaded them, with the following results:
- 1.4565" average
- 0.0065" extreme spread
- 0.0022" standard deviation
- 50% of base-to-shoulder measurements within 1 SD, 100% within 2 SD
After firing, I took another random sample of 10 cases of the same 50 and got these results:
- 1.4595" average
- 0.0040" extreme spread
- 0.0015" standard deviation
- 70% of measurements within 1 SD, 100% within 2 SD
So obvious conclusions: The cases are ~3 thousandths longer after firing, and a little bit more consistent with each other.
Here's what I'm not sure of: is an ES of 4 thousandths here fairly normal, or is it a decent indication that I didn't have enough pressure to effectively/fully fireform to chamber dimensions? SD of 1 and a half thousandths with 70% of cases within that distance from the average is pretty good I think, but the ES seems high. This comparator is new for me, but I would estimate that my measurement error here is probably no more than about half a thousandth on any given case.
If these figures seem more or less like "yep, those fireformed as you would expect," then I'll resize, bumping the shoulders back so they land at about 1.458", then anneal and load for accuracy with my heavier match bullets and proper rifle powder.
If they seem inconsistent enough to indicate they didn't fireform properly, then I suppose I could try again, maybe with a hair stiffer load (9.1 or 9.2gr of AA#2 instead of 8.5). The quickload data provided earlier in this thread indicated my pressure was around 37k here, and would hit 45k at 9.5 grains. With margins of error using imperfect simulated pressure data, I don't know that I'd want to push much closer to a "max" load of pistol powder.