I noticed my P17 was giving lower than expected velocities when I was shooting at ~3000 ft elevation. My first thought was I may need to refresh the o-rings. On returning to my usual 1000 ft, the velocity returned to normal:
.
1000 ft: 410 fps
3000 ft: 381 fps - 93% expected velocity
.
I considered atmospheric pressure:
1000 ft: 733 mmHg
3000 ft: 681 mmHg - 93% of the pressure
(ref: http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html )
.
I know there is a complex nonlinear relationship between pressure and velocity in an airgun, but for small changes the differences would be approximately linear.
This observation makes sense considering the SSP is compressing "less" atmosphere at altitude. I imagine this would affect spring piston airguns as well in a similar way.
.
The thinner atmosphere should also exert less drag on the pellet enhancing the external ballistics and partially, fully, or over compensating for the reduced velocity.
.
Prior to making this post I looked around for similar observations and found this excellent introductory article which mentions both points:
http://www.crosman.com/get-hunting/airgun-ballistics
.
So do these effects cancel each other out? Do you precision shooters consider barometric pressure when shooting Field Target for instance?
.
1000 ft: 410 fps
3000 ft: 381 fps - 93% expected velocity
.
I considered atmospheric pressure:
1000 ft: 733 mmHg
3000 ft: 681 mmHg - 93% of the pressure
(ref: http://www.engineeringtoolbox.com/air-altitude-pressure-d_462.html )
.
I know there is a complex nonlinear relationship between pressure and velocity in an airgun, but for small changes the differences would be approximately linear.
This observation makes sense considering the SSP is compressing "less" atmosphere at altitude. I imagine this would affect spring piston airguns as well in a similar way.
.
The thinner atmosphere should also exert less drag on the pellet enhancing the external ballistics and partially, fully, or over compensating for the reduced velocity.
.
Prior to making this post I looked around for similar observations and found this excellent introductory article which mentions both points:
http://www.crosman.com/get-hunting/airgun-ballistics
.
So do these effects cancel each other out? Do you precision shooters consider barometric pressure when shooting Field Target for instance?
