Elevation pressure differences

[tonyf3]

I'm shooting a 7mm Rem- 72gr retumbo, 180 Hybrids, Win Brass and CCI250s.
I've been shooting virgin brass run through a .306 bushing. This load was developed using virgin and once fired brass at about 5000ft. This will group .6-.9 inches at 300 yards

I reloaded some once fired brass by full sizing it and using the same recipe but here's where things get confusing for me.
At sea level I'm getting sticky bolt lift, small ejector marks and my groups open up.
If I shoot the virgin brass with the same recipe it groups perfectly with no pressure signs.
I drove up the hill to 6500ft and shot the once fired brass and the pressure signs are gone.
It appears i'm getting high pressure at lower elevations with previously fired brass.

Has anyone run into this and have an explanation?

 

[Hawk in WY]

You will get higher velocity at higher elevations because the air is less dense.

Pressure should be the same unless it was colder at the higher elevation.

Higher temperature definitely leads to higher pressure with most, but not all, powders.

My guess: It was colder at the higher elevation.

 

[Mikecr]

2 changes at once (at least): elevation, and sizing.
There is no reason for elevation itself to affect internal ballistics.