300WM - factory ammo "overpressure" signs

[klarson]

Overview: Doing load development for Seekins Havak Pro Hunter PH2 in 300WM, brand new with maybe 60 rounds of factory ammo through it. Rifle owner gave me the once shot brass after he zeroed it. I noticed the once shot was quite a bit more difficult to resize than I am use to, to the point were I was getting small brass shavings near the belt. Never had this issue with my 300WM shot brass.

Range: Did a work up of H4831SC from 68.7 to 70.5 with Nosler 200AB. At 68.9 had partial case head separation and a barely flattened primer @ 2752 fps. I immediately stopped there. I shot a factory Hornady ELD-X 200gr just to check since I've never shot this rifle before. Primer was extremely flat and a nice shiny ring above the belt. This leads me to believe the headspace is off by quite a bit, but wanted to double check with people more experienced than I.

Measurements:
Hornady ELD-X 200gr factory round
Datam at shoulder: 2.256
Diameter just above belt: 0.506

Once shot
Datam at shoulder: 2.274
Diameter just above belt: 0.516

I attached the pictures of the once fired FACTORY round.

Any thoughts and advice? I appreciate your time.

 

[918v]

My .300WM is like that: fat and long chamber. But the brass lasts well, even Hornady. The reason your brass is hard to resize is because your die is too tight for brass that fat. Most .300WM chambers allow the cases to expand to .512-.513” and a typical FL die brings that diameter down to .511-512” . But in your case the base diameter is excessive. To overcome this problem I roll size my brass first, then body size, then collet neck size.

As far as case head separation, mine doesn’t do that. The .300WM headspaces off the belt, not the shoulder. If your chamber is allowing the brass to separate like that then maybe you should have the headspace checked with hs gauges.