Reloading elevation (pressure inside casing) variance

[Tonyap]

So i have been working with a rifle i received from weaver rifles back in december. He guarantees 1/2 moa or money back. Its a deviant action in 338 lm. I will skip all the load data garb and get straight to the point. I sent the gun back to kevin. He sent 3 target pictures of one hole and said they were all 3 shot groups. Said the gun didnt like 300 gr berger hybrids etc. said the gun liked the 265 gr barnes lrx bullets with 92 gr h1000. He loaded me 3 bullets and sent it all back.

I loaded up the same bullets and weighed everything out. Measured bullet total weight and made sure i matched everything. Col etc...

Well my rounds were so freaking hot i had to knock every single one out of the chamber with a rod. Max load from barnes was 85.8 gr. I then shot kevins 3 bullets. No probems anywhere. My group was also right at 1/4”. So i started reducing the load. Finally at 88 grains the pressure signs disappeared and the bullets were stacking under 1/4”. I majored in EE in college and have basic physics. I remembered pv=nrt and remember opening a plastic bottle at kevins house near pikes peak and closing it. When i got home to 2500 feet elevation the bottle was obviously crushed as the atmospheric pressure outside is greater than inside as well as less oxygen etc.

So thats my experience and heres my question.

If you load a bullet at 8000 feet elevation and then ask a guy at 2500 feet elevation to load the same bullet and use the same powder do you use the same anount of powder or much much less due to atmospheric pressure and oxygen differences because theres a ton of difference between 8000 feet and 3000 feet. There was a ton of difference between these two loads.

Can someone with actual knowledge about this please chime in?

Thanks in advance guys.

 

[Texas10]

As far as oxygen is concerned there is no difference as the propellant has its own oxidizer. As far as elevation is concerned, that will have an effect on the exterior ballistics, but not interior, at least not in any measurable way.

The missing piece of relevant information in your post is what brass was used by you and your GS. Or more specifically, what is the volume of your brass as compared to his, and now much headspace are you providing in your load compared to his.

88 grains of H1000 was a great load for 285 ELD-M's in my buddies 338LM. Shot a mile, no problem.

 

[Ned Ludd]

Component variance, especially Lot-to-Lot variance in powder can make a big difference. I've had different Lots of powder vary by as much as 75 to 100 fps for the same charge weight, which translates to 1.0 to 1.2 gr difference for the cartridges I reload.

The real key is to reproduce the velocity his loads generated, regardless of the charge weight that requires. If you reproduce the same velocity, chances are good the load will tune in comparably.