Hello all,
      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?  

Any thoughts?

It's not uncommon for the shoulder to get bumped back when being fed in a semi-auto rifle. Have you tried measuring new ammo to get it's headspace reading?

I've used a different system for .223, .308 and .30-06. I purchased Mo DeFina's excellent headspace gages over two decades ago, long before Stoney Point or Hornady offered one. His is very similar ti RCBS's dedicated headspace gages, only he uses an actual chamber to measure the headspace, not just off the datum line. I set my headspace back -.004" from the fired measurement.

If your tools are clean, you're getting consitant readings and aren't holding anything on an angle, I would say you are very close to where you want to be. You might try 1.456" to be sure.

Which headspace blank are you using? It comes with five, right?

Thanks for the reply.

How much is too far when setting the shoulder back.  How many thousandths is going to overstretch my brass?

This makes me want to go measure my case gauge.

1.455", that's what I size all my brass(.223) to for head space, that's .003"(1.458" fired) bumped back and fits a Wilson and Dillon case gauge perfect so that # should be perfect and only another .0015" to go before you get there.

If your rifle is over gassed or your shooting hot loads the rifle will start extracting the case under pressure therefore pushing the shoulder out more than it would have been if extraction was slower and the pressure in the case had more time to "settle" so the case would start relaxing instead of still stretching(ejector swipes are a good sign of this). This "could" be making your head space look longer than it really is, but if the case really was .003" to long for the chamber even after sizing it would be a bitch to extract with the charging handle even though the chamber pushed the shoulder back, it would be stuck most likely to a point.