PCP velocities at altitude

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camtuning

Member
Feb 7, 2019
71
20
NM, United States
Just picked up a Vulcan 2 .25 last Friday and have really been enjoying it.
Talon tunes shot it in Florida and logged 921-938fps with JSB's, and that is what I have been using in Strelok pro for the last 4-5 days. Today I decided to shoot it through my chronograph and I was surprised to see 892-908fps with the same pellets. I live at ~5200' above sea level, and our atmospheric pressure is typically 11.9-12.1 psi vs ~14.5 at sea level. The articles I've read online suggest that PCP rifles should not lose velocity at altitude the way that a springer or multi pump does. Does anyone have experience with this?
 
The air density changes, why wouldn't the speed change. Maybe what your finding in your reading. The author as tested in a narrower change and noted other variations to have bigger implications then air density at different altitudes. If your testing the difference between 100' ASL to 1200' ASL. The change was unmerited. Where the FPS could be impacted as much by other forces such as pellet tolerance.

Now your testing from 100' ASL to 5200' ASL, Now thats a real test. Your testing a 30FPS difference. If tested at 10000 maybe there could be expected loss of 60FPS total. With the testing of 1100' difference and only being 25% of your altitude they may. Have only seen 7-8FPS change. That's with in the expected normal pellet to pellet difference.

What, I'm taking from your findings is. yes there's a change. In your application there is a 5FPS drop per +1000 of elevation change.



Good find
 
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I live at 5600 and shoot field target matches at 1700 feet in the winter and 7200 feet in the summer. Springers are drastically effected by the elevation but PCPs don't seem to have the swings of fps that I see with the springers. My scope clicks figured at home, work just fine down at 1700 with the PCP.

I think there's alot going on here though, theoretically. To get chairgun and Strelok to agree to actual trajectories at my home elevation, I have to enter higher than normal ballistic coefficients. That holds true with all my pcps. 

My simple mind goes with the fewer air molecules for the pellet to interact with on it's way to the target up here in the thinner air idea. Less "resistance" means a higher bc and a flatter shooting gun. 

Springers are completely backwards though, the thinner air means there is less air in the compression chamber to pressurise when the piston slams forward. I've often thought that the thinner air in the chamber of the piston gun would be counteracted by the thinner air in the path of the pellet and it'd be a wash, but, in real life, I actually lose lots of fps with springers living up here. 

I'd love to hear some real silence behind all this though, instead of just my anecdotal findings. 
 
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Hmm, technically with lower air density, your velocity *should* increase...just like an airplane can fly much faster at high altitudes compared to sea level, not just for structural reasons but also for the simple fact there is less air resistance due to lower density, also the reason we can obtain absurd velocities in space.... For example, if I change my altitude in sterlok pro from 5200~ to sea level, it calculates there is MORE drop, for this very reason.



Not just that, but there is technically less weight of air in the barrel resisting the projectile as its pushed down the bore...however very marginal, its still in the favor of higher velocity opposed to lower...



What distance were you shooting from your chronograph?


Theres a few possible explanations provided its not simply chronograph distance....but I am hard pressed to say its due to increased altitude. 



-Matt


 
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I just shot at sea level weeks back and I leave at 6000. I usually don't see much changes, due to elevation between 2000 and 6000. But, temperature made a huge difference from shooting at 10 - 30F to 80F. I was surprised by how my curve was so much linear up to 60 yards. normally between a 30 and 50 yards shot I usually have to compensate 2-3 MOA in my cold mountains. When in Puerto Rico few weeks back no correction was needed or less than half MOA. I am not saying that the elevation doesn't impact the ballistics, but temperature had much more effects from my experience.



Edit: In my case this time, after more reflection, I had both Temp and elevation playing for me. I do agree with the previous comments that high elevation or temp means lower air density thus lower drag
 
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Temperature will certainly have an impact on velocity, even more so when regulated...any large environmental swings tend to have an impact, from humidity, temperature to elevation and of course, wind conditions, even dense rain/snow will of course have some effect. The distance between you and the target is technically a medium, and any alteration of that will of course effect trajectory. 
 
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well everything affects your shot , the higher you go the faster and better bc will be , but you talking a few percent not like 50% differences

Ackuric made a true statement temperature makes alot more difference on your poi up to 20% the hotter it is the faster it would go

here is example

if i set my 22 cal pcp at 868fps and it is 15 degrees here

if I took it out in July when it is it would be like 902 fps


 
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Hmm, technically with lower air density, your velocity *should* increase...just like an airplane can fly much faster at high altitudes compared to sea level, not just for structural reasons but also for the simple fact there is less air resistance due to lower density, also the reason we can obtain absurd velocities in space.... For example, if I change my altitude in sterlok pro from 5200~ to sea level, it calculates there is MORE drop, for this very reason.



Not just that, but there is technically less weight of air in the barrel resisting the projectile as its pushed down the bore...however very marginal, its still in the favor of higher velocity opposed to lower...



What distance were you shooting from your chronograph?


Theres a few possible explanations provided its not simply chronograph distance....but I am hard pressed to say its due to increased altitude. 



-Matt

Click to expand...

I am shooting from just 4-5" in front of the chrono. I shot again this afternoon and made sure I was level with the chrono and got some better results. I shot 10 shots ranging from 913.5 to 923.9. It's still a littler lower than Talon but I talked to Tony and he said it's within normal variance.
 
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Just picked up a Vulcan 2 .25 last Friday and have really been enjoying it.
Talon tunes shot it in Florida and logged 921-938fps with JSB's, and that is what I have been using in Strelok pro for the last 4-5 days. Today I decided to shoot it through my chronograph and I was surprised to see 892-908fps with the same pellets. I live at ~5200' above sea level, and our atmospheric pressure is typically 11.9-12.1 psi vs ~14.5 at sea level. The articles I've read online suggest that PCP rifles should not lose velocity at altitude the way that a springer or multi pump does. Does anyone have experience with this?
Click to expand...

I think muzzle velocity should remain the same at any altitude.
 
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Click to expand...

I am shooting from just 4-5" in front of the chrono. I shot again this afternoon and made sure I was level with the chrono and got some better results. I shot 10 shots ranging from 913.5 to 923.9. It's still a littler lower than Talon but I talked to Tony and he said it's within normal variance.
Click to expand...

Could I suggest that you move the muzzle back to about 18 inches from the chrono. Again be sure to align the barrel parallel to the sensors. Let us know the result please.

Kind regards, Harry.
 
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