I had my Impact tuned at 990fps at sea level. I went to 3000 feet and it was shooting at 965. My first thought was, the air up here is thinner, so the air in the barrel is thinner, so the slug is moving through thinner air. So shouldn’t the projectile be moving faster? The air in my bottle isn’t any thinner. My reg is at 150 bar at sea level, and it’s at 150 bar here at 3000ft. I’m clearly missing something.
I’m sure this has been discussed elsewhere, but I searched and wasn’t able to find anything discussing the physics. Feel free to send links to any discussion or articles that break it down.
**EDIT:
ADDITIONAL INFO (should have included this in original post)
Gauge Used: Huma Air 28mm Digital Gauge https://www.huma-air.com/Digital-Mini-Pressure-Gauge-for-FX-Impact-28-mm
ATMOSPHERIC CONDITIONS WERE MEASURED WITH a device called SENSOR PUSH https://www.sensorpush.com and the iPhone’s built in barometric sensor and GPS for elevation.
Elevation change: +3000ft
Temp Change: -8 Degrees F (60F to 53F at 3000ft)
Humidity Change: 7% (54% at sea level, 47% at 3000ft)
BP change: -3.8 inHG (30.5 inHg at sea level, 28.2 inHg at 3000)
CHRONOGRAPH: **The same chronograph: FX Pocket Radar Chrono) was used at both elevations. https://utahairguns.com/fx-pocket-chronograph/
POI CHANGE: -.75” (far zero: 50y) Point of impact decreased by about .75” at 100y, which is what I calculated would happen with that decrease in FPS, so I don’t suspect the chronograph was reading “off” at either of the elevations.
I’m sure this has been discussed elsewhere, but I searched and wasn’t able to find anything discussing the physics. Feel free to send links to any discussion or articles that break it down.
**EDIT:
ADDITIONAL INFO (should have included this in original post)
Gauge Used: Huma Air 28mm Digital Gauge https://www.huma-air.com/Digital-Mini-Pressure-Gauge-for-FX-Impact-28-mm
ATMOSPHERIC CONDITIONS WERE MEASURED WITH a device called SENSOR PUSH https://www.sensorpush.com and the iPhone’s built in barometric sensor and GPS for elevation.
Elevation change: +3000ft
Temp Change: -8 Degrees F (60F to 53F at 3000ft)
Humidity Change: 7% (54% at sea level, 47% at 3000ft)
BP change: -3.8 inHG (30.5 inHg at sea level, 28.2 inHg at 3000)
CHRONOGRAPH: **The same chronograph: FX Pocket Radar Chrono) was used at both elevations. https://utahairguns.com/fx-pocket-chronograph/
POI CHANGE: -.75” (far zero: 50y) Point of impact decreased by about .75” at 100y, which is what I calculated would happen with that decrease in FPS, so I don’t suspect the chronograph was reading “off” at either of the elevations.
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