I'm running into an issue with my FL resizing that I'm not sure what to think.
My goal is to set my FL resizing die so that my 223 cases are resized with the shoulder being set back the appropriate amount for an autoloading rifle cartridge. I have read 0.002 to 0.004 below the chamber headspace as measured at the shoulder.
Using a Hornady / Stoney Creek Headspace gauge, I measured 20 cases from each of my two DD AR15s. The cases measured had been loaded to 5.56 max pressures, so I figured they would be a good imprint of my chamber headspace. I got 1.4610 plus or minus 0.001 with both rifles being almost identical.
I set my FL sizing die and resized a few cases with the Hornady gauge measuring the shoulder at 1.4570.
I didn't think I could load an empty brass so I put it in the chamber and dropped the bolt on it. After extraction the gauge showed that they chambering bumped the shoulder back to 1.4540 and it did it every time I tried it.
I wondered if this is because I am dropping the bolt on a piece of brass which is already in the chamber, so the extractor is hitting it hard and pushing against the shoulder. As I understand it, you are not supposed to drop a bolt / slide on a round that is already in the chamber. I figured I was only doing this once or twice so it wouldn't be an issue.
So I got another few brass that were set at 1.4570 and put a 55gr hrndy FMJ BT in them w/o powder or primer. I put them in a magazine and chambered each of them. They showed the same shoulder bump to 1.4540.
Is this normal? Am I wrong on my initial measurements of the chamber? Should I just set my resizing die all the way down to the shellplate and go with that? Should I try to set it at 1.4540 and see what happens then?