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11 October, 2010

Keep Powerlets in Your Pocket, Gain 20fps Muzzle Velocity With Your QB78

Here's some test results that show why you should keep 12 Gram CO2 Powerlets in the pocket of your jeans before loading them into your QB78 family air rifle. Oh yes, and then wait for 3 minutes before beginning to shoot. It can make 20fps difference to muzzle velocity!

I made 10 tests, each time loading a QB78 air rifle with two full standard 12 Gram Powerlets. In every case, I had warmed the Powerlets in the pocket of my jeans for about 30 minutes beforehand.

The table below shows (from left):
1) The ambient (room) temperature.
2) The initial temperature of the gun before loading Powerlets.
3) The temperature of the gun immediately after the warm Powerlets were loaded.
4) The temperature of the gun immediately after piercing the Powerlets.
5) The temperature of the gun after waiting 3 minutes.

On average, the guns started at approximately room temperature - 67 or 68 degrees.

After running this test a few times, it occurred to me that I should be recording the temperature increase of the gun after I loaded the Powerlets that had been in my pocket. Duhhh...

So, we can see that the temperature of the gun rose by about 7 degrees F immediately after loading with the warm Powerlets.

Piercing the Powerlets caused the temperature of the gun to fall by 10 degrees. Gun temperature stabilized at about ambient temperature again after 3 minutes in ambient temperatures of 65 to 70 degrees F.

So, what do we learn from this data?
- Keep Powerlets warm before use.
- Wait for at least 3 minutes before starting to shoot your gun.

The reason for all this interest in gun temperature is that the muzzle velocity of QB78-family air rifles - like all CO2-powered guns including the Crosman 160 - changes with temperature. For the QB78, this change is 2fps per degree F. So we see that just piercing the Powerlets in your gun causes the temperature to fall by 10 degrees F. That represents a FALL in muzzle velocity of 20fps!

For those of us wanting the best performance from our Q78s, that 20fps makes a big difference, not just to muzzle velocity, but also to the point of impact.

Doubtless, the results would be different in higher ambient temperatures (and the differences less marked), but 70-ish has been about the best we have been able to manage in Up-State New York recently. Conversely, I would expect the effect to be more marked at lower ambient temperatures.

The gun temperature readings were made using the Archer Airguns Temperature Monitoring Kit for QB78.


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This blog shares information, ideas and knowledge about air rifles. It compliments the information Stephen publishes on the Archer Airguns website, on YouTube and the Chinese Airgun Forum.

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