Shooting/resizing Peterson 7PRC brass

[Charlie-NY]

OK - I'm just getting started with my 7PRC in a new Bartlein bbl. The unfired Peterson brass measured 1,8905", on average, when measured from base to datum point using a .420 bushing and the Hornady Headspace comparator tool.

I loaded the brass without resizing it. I used a 180 Berger and 61gr H1000 over a Fed 215M. I fired 5 rounds only.
Went home and noticed a few things that I didn't expect.
1. Barrel is producing many blue/green patches and is still copper fouled.
2. Peterson brass shrunk and now measures 1,886" to the datum point, a reduction of .004" in headspace length

I set up the Redding Type S sizing die but did not want to push the shoulders back as this point. My initial adjustment resizes the case and resets the shoulders back to 1,886". Cases easily slid into the chamber.

Any thoughts or comments?

 

[Zero333]

The shrinking is due the shoulders blowing out to the chamber's dimension.
Not unlike taking a balloon, grabbing it by it's sides and stretching it sideways. The top to bottom measurement of the ballon will shorten.

This is normal with some brass and chambers. By the second firing it will not shrink the HS anymore and start to move it towards the shoulders of the chamber.

Next time you load it, I would seat the bullets so they're in the lands. This will help some so the brass body may stretch evenly, and not just in one spot.

 

[Coyotefurharvester]

The other possible, angle of the shoulder changed when fired. The comparator body will then make contact at a different spot. I would use the spent primer method and find the bolt to brass clearance for both the new and fired brass(not resized), see if the was any lengthening after firing. You will have a better idea of the headspace you are dealing with.