Effects of heat on velocity and accuracy
I was doing some range work today to get precise scope dope on my Leupold Holland VXIII LR 30 MM which is on the 400 these days.
I lay the rifle in the bags and went down range to staple up a target at 50, shot 10 shots, then moved the target to 80 and shot 10 shots, then 90 and then 100 moving the target each time and shooting 10 shots. I made the trip down range each time I shot ten shots at each range to move the target out further, all in all it took an hour and ahalf to do this.
I refilled to 3000 PSI after each 10 shots.
The weather all summer here has been cold and rainy, but today it was in the 80’s. My rifle was in the sun on a dark green hood cooking.
I had recently turned it up a bit to around 950FPS from 905 FPS with 18.1 JSB’s and I was doing all this because I wanted to get new range settings.
I noticed I was shooting far lower and suspected the heat was increasing the tank pressure from 3000 PSI to a higher level and giving me a bit of valve lock.
I recently broke the gauge on the 500 carbine, so I had the remaining gauge on the carbine and not on the 400, so I have no idea what it was reading.
But I shot this group with a 14.8 inch drop at 100 yards with a 50 yard zero, far more than it should have been if it had been doing 950 FPS.
Once back at the house, Charigun told me I was doing 860 FPS to have that trajectory for 50-80-90-100 yards.
However this is the group, the wind was pushing it 2 inches to the right and gravity was pulling it 14.8 inches down my MOA reticle.
I should have shot another and if it fell into the main group called it a 5 of 6 but I did not realize how tight it was. 4 shots in just over 1/2 inch, which is why I prefer the 400 over the 500 carbine past 80 yards, which by the way is still in my closet, not going anyhere.
So to check my suspicions about the heat and velocity loss, I took the rifle in the house for an hour at 73 degrees. Then went out and ran it over the chrony. I got three shots in the 950FPS realm once again.
Then I put the rilfe in the direct sunlight for only 20 minutes at 80 degrees, and when hot to the touch, I shot it over the chrony once again.
It averaged 900 FPS, so I dropped 50 FPS in just 20 minutes of direct sunlight with a 3000 PSI fill. If I had done this with a 300 mag, I would have gotten a higher velocity.
The thing I love about PCP’s in regards to other guns is that there are so many different variables to experiment with and learn about..
Regards,
Roachcreek
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If you set a co2 gun out in the sun it will valve lock also.
RC
My guess is that if you got the temperature of a CO2 tank high enough, you might encounter the same results. But in the range that CO2 is used I doubt you’d see the “vapor lock” issues. To push CO2 pressure high enough to see that sort of effect it would probably be much too hot to be outside shooting.
The more I see this sort of thing, the more I think that a regulated PCP is really the only solution.
have to agree, a regulated gun wouldn’t have the same problems. but, i’d bet the farm that shot count would go up.
as far as getting older, my dad always said the day you stop learning is the day they throw dirt in your face. that’s why i always listen. you can almost always pull a gem out of a conversation, that way.
I think the lower velocity is a particular phenomenom that applies to unregulated PCP’s that are filled very close to their maximun fill level.
If you had lets say 2400 PSI in your Condor tank and your maximum fill level was 3000 before valve lock occurred, you would perhaps get higher velocities.
I would also expect that my 500 carbine would not suffer from this lower velocity because it is regulated.
RC
Yeah, that’s the joy of being a human and getting older, as long as you can make a turd and get a hard on, or buy one if need be, you have the brain power to learn something right up to the day you start rotting out a coffin liner.
You just have to get over the know it all folly of youth and realize, “wonder of wonders, I could be wrong, maybe I should look at this differently.” 😛
Regards,
Roachcreek
WOW! i always assumed that higher temps would yield higher velocities, due to the heated air being ” thinner”, so to speak. guess it’s just the opposite of CO2. ya learn something new every day, don’t ya.
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Ah, so they do then. I wonder how hot that CO2 bottle gets in the sun, and what the internal pressure climbs to.