For some years after their introduction, many manufacturers promoted gas ram air rifles as being unaffected by changes in temperature. The FPS would be pretty-well the same at any temperature, they said.
Is that true? Well, on the basis of some testing I’ve undertaken, the answer is definitely “no”. At least in the cold weather we have here in up-state New York.
I’ve found that here is definitely a change in FPS for gas ram air rifles at different temperatures. And it’s more than you may have thought!
To find out what the change in FPS can be with gas ram air rifles shooting at different temperatures, we used a new .177 caliber SIG ASP20.
We shot the ASP20 at a temperatures of both 20 degrees F and at 63 degrees F. In each case, the gun was allowed to “season” at the ambient temperature for several hours before shooting. This meant that gun and ambient temperature were definitely at the same.
Also, we shot the gun slowly – about every 30 seconds – for each test. This was to avoid any effects from the ASP20 heating-up as it was fired. We took 10 shots at both temperatures for each of the six types of pellets. Total 120 shots.
So what did we find?
The answer that – taking the SIG ASP20 as a representative of gas ram air rifles – the gun shot faster, on average, by 1.28 FPS per degree F at the higher temperature.
On average, that means approximately 55 FPS difference when the gun was shot at 20 degrees F and 63 degrees F. That is very definitely enough to make the point of impact on the target very different at most ranges.
So if you’re shooting gas ram air rifles, either on the range or hunting, make sure that your gun is sighted-in at approximately the same temperature as for that critical shot. If not, you could miss the target just due to the change in temperature!
Here’s a chart showing how the Muzzle Velocities changed with temperature for different .177 caliber pellets:
And here’s the average…
So if you’re shooting gas ram air rifles, either on the range or hunting, make sure that your airgun is sighted-in at approximately the same temperature as for that critical shot.
If not, you could miss the target just due to the change in temperature!