Velocity vs. Pellet Drop?

[Poorman Plinker]

Greetings,
I know there is no one answer to this question, but I would appreciate any feedback on the topic of velocity drop and drop in point of impact. The question:
What would be an acceptable (reasonable) variation in velocity for a target gun? It would be helpful if you would specify pistol of rifle, 177 or 22, 10 meter or long range. For example 15fps, 177 pistol at 10 meters.
Thanks in advance. 

[arkmaker †]

You're right, every rifle/pistol varies from my experience, but here is the shot group from the Challenger 2000 12g Co2 Rifle that I just worked on. I noticed a drop in POI @ 10M when the velocity dropped below 400 fps, which was about shot 85 if memory serves me correctly. I only show 80 shots on this chart. Now that I know what this rifle does, I stop at 75 shots to assure that POI remains where I want it. My thought is that if you do not have a chrony, all is not lost when determining POI with Co2 or HPA. The chrony is the only way to put numbers (fps) to the shots taken, but without a chrony, you can just count the number of shots at a given distance until the POI drops to an unacceptable level and then back off 4-5 shots the next time you pop in a fresh cart. It's that easy. (but people like numbers ;-)

2nd photo shows the groups I am willing to live with. Sorry, I can not find the one showing the POI drop off. (this rifle POI drops about 1" to 1-1/2" per shot when it does start dropping)

[mr007s]

Would I be correct in saying that the shot count and drop will differ when the temp rises and falls, or just the fps?

[arkmaker †]

Would I be correct in saying that the shot count and drop will differ when the temp rises and falls, or just the fps?

I think that would be correct. That's why with the Challenger, I stop at 75 shots (to take all things in to account, without having to think to much about it). But first, I had to run a couple of carts and count shots to get a feel. I figure that when the weather turns cooler, the count may drop even lower, but compared to the other 22xx guns I have, I am ok with that. I could compare it to the gas mileage in my truck, summer vs winter, only reversed. I get better gas mileage in cooler temps.

My Dad had kept that old Shiloh pistol that I bought back in the late 70's or early 80's? Really, it is a dog of a target pistol, but because he shot it so much, he could tell when POI was going to drop without much thinking. It truly amazed me at how well he could shoot that puppy.

There sure are a lot more factors to deal with when using Co2 as compared to pumps or PCP's!

[Poorman Plinker]

Wow Arkmaker, nice presentation.
If I am reading it right you do not see a drop in poi until a variance of 18% - 490/400. However you like to shoot in a range of 4% - 490/470.

I like the metaphor of the gas mileage and pellet velocity, but I think a novice might take it wrong. If you could isolate the temp of the co2 cart and the humidity of the air I think temp would have little to no effect on pellet flight (just speculating). However cold weather effects the evaporation of the co2 just like it effects the evaporation of the gas in your gas tank. So reduced temperature = reduced co2 pressure and so reduces pellet velocity.
Depending where a person lives often the cold season is also the rainy season and humidity increases causing more drag on the pellet. Though the muzzle velocity would be only slightly effected long range flight could produce a noticeable change. Cold to an extreme will actually dry the air and humidity will decrease and the air is drier. So it is a complicated question when it comes to temp and co2 pellet performance.