300 WM case head expansion...

[IdahoSharpshooter]

So, I take my tube gun 300 WM out after lunch, just playing at 200yds. I had half a box of Barnes VOR-TX 165 gr tipped TSX BT left from a hunt. I fire the first shot, and recoil is WOW! I have trouble with bolt lift. Load a second round, and results are identical. Ditto for the third. Group is right at 2", and an inch higher than my 190gr Sierra MK hand load from the day before. Recoil is heavier as well.

Drive home, grab my dial calipers and start measuring.
1. primers show no flattening or cratering, and the old scrape the thumb nail across the case head test shows no raised area. It looks normal. There is NO extractor/plunger mark on the case head.
2. I measure the other 11 factory rounds, and they mike .5055"-.5070" just in front of the belt.
The three fired rounds mike .5085"-.5090". To me, that seems normal, but the heavy bolt lift is a bit disconcerting.

This is a Remington factory barrel, 700 LA, and it head spaces normal with my gauge.

Any thoughts on what would cause the heavy bolt lift if every thing else is normal?

thanks to all,

Rich
Sua Sponte

 

[dkhunt14]

Possibly a carbon buildup in the throat area. That can cause the problem you have. MATT

 

[IdahoSharpshooter]

Thanks, I thought that, but the barrel was cleaned thoroughly, and only nine rounds thru it before these three.

 

[jdh47]

My guess is that the empties are going to measure the same as the ones previously fired from that box. If the extraction timing is off a little it could account for it however you haven't mentioned that.

Other than that I have seen ammo that was carried in a truck that was jostled around and got hot before change burn characteristics. That can and does happen. It happened to a friend of mine and it blew some primers in a 6mm.

 

[IdahoSharpshooter]

It had been to RSA and back on airplanes.Weather was only 100-115 for the ten days.
I asked a friend, and he suggested measuring the belt. FGMM brass, once-fired and neck-sized in Wilson die, belts of that fired brass, unfired Barnes factory ammunition I shot, and the fired are all within .0007".
A sunny and very cold day, it is 11:31pm, and the current temp is +14-degrees.

Further testing may have to wait a week or so.

...?