Nitro Piston Temp Sensitivity

I have a Remington Tyrant(Crosman Nitro variant) that I bought last fall. I believe it was doing ~920 with FTT’s. Then in the winter, I chronographed it at ~880 FPS. Today it’s 85 degrees outside and the thing is doing ~980 FPS with FTT’s, and almost 900 with Barracuda Hunters.

Is this normal? Gas inside the ram expands/contracts and gains/loses pressure? Run Barracudas in the summer and FTT’s in the winter?

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I can tell you that I experience a similar issue with my asp20. Goldenstateairgunner (Wayne) had some spreadsheet regarding this issue in his asp20 (in .22 grrrrr)

I wonder how much that effect could be mitigated with the (slightly) adjustable into piston that Vortek is working on...

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I'd be interested to find that out. The change in fps alters the POI, so it bugs me...

Sorry to get overly technical, but yes the pressure inside the ram will change with temperature. The pressure will be proportional to the absolute temperature. Going from 60 F to 80 F the increase in (absolute) pressure will be about 4%, which means the velocity will increase by about 2%. So a gun shooting 880 fps at 60 F will theoretically be shooting almost 900 fps at 80 F. There is nothing that can be done to correct this that I am aware of other than to make some kind of adjustment table. Never really thought about it before.

I suppose what could be done is to make a series of index marks below the turret to indicate the zero offset at different temperatures. Not perfect, but the only practical solution I can think of.

I have a Remington Tyrant(Crosman Nitro variant) that I bought last fall. I believe it was doing ~920 with FTT’s. Then in the winter, I chronographed it at ~880 FPS. Today it’s 85 degrees outside and the thing is doing ~980 FPS with FTT’s, and almost 900 with Barracuda Hunters.

Is this normal? Gas inside the ram expands/contracts and gains/loses pressure? Run Barracudas in the summer and FTT’s in the winter?

Click to expand...

I can tell you that I experience a similar issue with my asp20. Goldenstateairgunner (Wayne) had some spreadsheet regarding this issue in his asp20 (in .22 grrrrr)

Click to expand...

Yes ! Gas spring airguns fluctuate with ambient temperature changes. The greater the temperature the greater the BAR inside the gas cylinder. Same principle when checking your car tire pressure on a 30 deg morning and then checking them again when it's 70 deg. I've noticed a increase in my Sigs cocking effort when it's warmer weather. This phenomenon is important if you hunt, or target shoot, with a gas spring airgun. Your point of impact will be different if you sighted in your rifle in hot weather and then hunt on a cool morning. Or vice versa. That's the one thing I like about coil spring airguns. Temperature makes no difference from what I've noticed. Barometer and elevation does. But that is true with gas spring guns as well.

I posted this in another forum. This info was based on a Sig ASP20.

https://hardairmagazine.com/reviews/do-gas-ram-air-rifles-give-different-fps-at-different-temperatures/