Shooting at altitude
by B.B. Pelletier
I recently spoke to a disgruntled airgunner about how he hated spring-piston guns. He was fed up with them! They never lived up to their advertising. In the course of the conversation he happened to mention that he lives at 10,400 ft. altitude.
Of course his springers don't perform! At that altitude, they can't get enough air to compress, and no amount of wishing can change that fact.
Testing airguns at altitude
Over the years, several tests of different airgun powerplants have been performed at varying altitudes. The best one I have seen, though it was a short one, was done by Howard Montgomery, the owner of Reno Airguns. It was published in The Airgun Letter in 1996. Howard tested several springers, CO2 guns and multi-pump pneumatics from sea level to 8,500 feet. He found that springers became weaker (shoot slower) with altitude, but it's hardly noticeable until you get above 2,000 feet. CO2 guns become more powerful (shoot faster) as the altitude increases, and multi-pump pneumatics get weaker.
Springers die when you go high!
Howard's test ended at a lower altitude than the man I spoke to. I would guess that his springers would lose up to 40 percent of their rated power at the higher altitude. Howard also reported that his more powerful spring guns (an RWS 36 and a Gamo 2000, which is no longer made) were making horrible sounds above 6,500 feet. That's probably because there wasn't enough air to cushion the piston when it went forward.
CO2 gets faster, but there is a catch!
Less air means less resistance to a pellet in flight. Hence, faster speed from a gas gun. Unfortunately, the higher you go, the greater the chance the weather will be cool, and CO2 dies in cool weather. So, indoors at altitude, yes - outdoors (most of the time) - no.
When the air is rare, multi-pumps have nothing to compress
Multi-pumps got slower as the altitude increased. There is less air for them to compress and store. That could be offset by pumping more strokes to compensate, but Howard stuck to the owner's manual.
Howard didn't test them, but PCPs will dominate at altitude. A PCP is less affected by cold than any other powerplant, and a PCP stores compressed air at a constant pressure. The combination of lower air pressure outside and constant pressure in the reservoir means that a PCP will just get faster as the altitude increases. This could be offset if the gun has been lubricated with oil or grease, and the temperature goes VERY low. Also, low temperatures cause a drop in air pressure inside the reservoir - just not the great drop we see in CO2.
So, for high places, the PCP is the powerplant to choose. If you go with a multi-pump, you'll have to experiment on how many extra strokes to give it.