Cases with inexplicably short headspace after sizing

[jumbopanda]

I was resizing a batch of twice-fired 300 Blackout brass that I had formed from 223 cases, and came across a very strange issue. Out of about 320 pieces, 4 had their shoulders bumped a LOT farther back than any of the others (0.010"-0.020" shorter). At first I thought that I might have over-annealed some of the cases, so I took a test case, purposely over-annealed it by heating it until it glowed orange, then sized it. Surprisingly, the headspace on that case did not change. So I'm really puzzled right now as to how a few cases could have had their shoulders pushed that far back. This stinks because now I have to go back and check headspace on all of the brass to make sure there are no more like that.

[LynnJr]

If the brass had excessive headspace out of the box and most brass does and your loads were light the firing probably didn't fully form those few cases.

[jumbopanda]

If the brass had excessive headspace out of the box and most brass does and your loads were light the firing probably didn't fully form those few cases.

I formed these cases from 223 and checked each piece with a case gauge before I loaded them last time.

[BLR81]

If you used a spray on case lube and resized while still wet, you may have gotten a hydraulic effect similar to the one that causes shoulders to dent.

[jumbopanda]

If you used a spray on case lube and resized while still wet, you may have gotten a hydraulic effect similar to the one that causes shoulders to dent.

Nope, they were dry.

[JagerDog]

I'd guess you had something impeding a good case gage read the first time around. Now that they've been fireformed they're sloppy in the gage.