Case shoulder dimension changing when chambering AR

[Bgd]

I did a test with a couple cases. I fired them in an AR with the gas turned off so I could get a case fire formed in the chamber without being extracted.
The fired cases measured 1.074 at the shoulder with the hornady case comparator.
I bumped the shoulder back to 1.071. I loaded them, chamberd them and the they measured 1.064 after I chamberd them. The COAL remained 2.184.
It does the same thing weather it is a standard buffer spring or a JP enterprise silent captured buffer spring. The BCG is mill spec.

I guess the AR slams the cartridge in so hard that it pushes the shoulder back?
Does anyone else experience this?

[2manyToys]

Did you measure that after loading, but before chambering? I've seen the shoulder move while seating bullets.

[Bgd]

Did you measure that after loading, but before chambering? I've seen the shoulder move while seating bullets.

I took the measurements on the finished round. it did this in more than one rifle so i am guessing this is normal?

[dellet]

Not uncommon, more so in an over gassed system.

Chamber a round by riding the bolt down with the charging handle and close it with the forward assist. Then check the shoulder length.

If it does not change, then it is all speed that is creating the problem. This is most likely.

If it does change, then the extractor springs may be tight. They might loosen up after a few hundred rounds.