Cold temperature affecting speed of projectiles

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@nervoustrig. Yes, they were from the same tin when I was shooting them two days ago. I had 15-17 fps speed lost and that had affected my groupings quite a bit especially further out passed 50 yards. My understanding with the slugs or pellets shooting is that for any given weight, the speed has to be in a certain range or number as well. At least that was my finding when I was tuning the 25gr Gen2 slugs to run at around 984fps. I got the best consistent accuracy out to 100 yards. My YouTube video (Pesting#194) demonstrated how efficient and accurate my Impact was before the temperature dropped recently.
 
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swNCsw said:
Doesn't the viscosity break down and thicken when dropping beyond a certain temp. Especially if it had been applied to thick it seems it would cause a substantial increase in sticktion for lack of a better word lol.
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Not only does the lube thicken but the Orings stiffen. Remember, the poppet shaft runs through a Oring that seals the forward area of the plenum and it already causes problems at normal temps for the first few shots. And if I'm not mistaken, rubber contracts as it cools which would cause it to grab onto the poppet shaft even harder. It would take more hammer to get the same valve opening.
 
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The only way I could think of to combat these instances back then was heating the pellet rifle lol .((( It worked to varying degrees)))) . A lil farmer creativity sometimes goes along way haha. I tried the lil hot hands heaters fastened to the pcp and I didn't quite like it but it was warmer . Then I tried a old par of battery powered socks I had been given as a gift lol . That was ok as well . As I increased in age I just quit hunting in 18* degree weather all together .

If I chose to give it a go or try in the future nowadays they sale rechargeable hand warmers that are tubular in shape that appear to be large enough to be able to slide over a pcp and modified . I would give one of those a go to keep everything warm and cozy haha. Just google hand warmers and you may find something that would work out . It would look alot nicer than 2 old battery powered socks strapped to it like I did :ROFLMAO:
 
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Max115 said:
@nervoustrig. Yes, they were from the same tin when I was shooting them two days ago. I had 15-17 fps speed lost and that had affected my groupings quite a bit especially further out passed 50 yards. My understanding with the slugs or pellets shooting is that for any given weight, the speed has to be in a certain range or number as well. At least that was my finding when I was tuning the 25gr Gen2 slugs to run at around 984fps. I got the best consistent accuracy out to 100 yards. My YouTube video (Pesting#194) demonstrated how efficient and accurate my Impact was before the temperature dropped recently.
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Herein lies your answer; video evidence so to speak. The stability of those slugs and the twist rate of the barrel dictates they be ran at the 984 fps range.

Tune it with higher regulator pressure to compensate for the cold temperature, and when it warms up adjust back with slightly less hammer spring pressure to keep the speeds the same. My Impact doesn't like the winter weather shooting either. Too many moving parts. I very lightly lube some dynamic parts/o-rings with Krytox GPL 205. It has Teflon in it and a good low temperature viscosity rating. Others, like the hammer weight I leave dry, but highly polish anything that has "witness marks" on it from rubbing.
 
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The speed of sound inside the barrel matters to performance independently of starting pressure.
The speed of sound determines the speed of expansion of a volume of gas. The molecules can't move away from one another any faster than their individual speeds due to temperature (which is the speed of sound). This is one of the reasons air gun efficiency drops as pellet speeds approach the speed of sound at normal temperatures - like the ones inside the barrel and tank. The speed of sound is much higher in a hot gas, like the one produced by gunpowder.
So maybe it's just that: the same amount of reg pressure pushes less on the pellet on a cold day, because the gas expansion is slightly less able to catch up with the pellet as the pellet approaches the same absolute final speed in the barrel. The change in the speed of sound is less than 0.5% between 0 and 20 deg. C - 1085 fps vs 1124 fps, but the change of the difference between this and the pellet (average) final speed is larger (assuming it matters). Maybe it's like having a slightly shorter effective barrel length, on a colder day.

There are ways around this problem, if it is a problem, on the air gun design side, but I think people mostly stumble on it by chance and not necessarily by design intent:


edit: just about everything I say in this post was debunked in a separate thread. Video is unrelated to cold weather performance.
 
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Airgun-hobbyist said:
Herein lies your answer; video evidence so to speak. The stability of those slugs and the twist rate of the barrel dictates they be ran at the 984 fps range.

Tune it with higher regulator pressure to compensate for the cold temperature, and when it warms up adjust back with slightly less hammer spring pressure to keep the speeds the same. My Impact doesn't like the winter weather shooting either. Too many moving parts. I very lightly lube some dynamic parts/o-rings with Krytox GPL 205. It has Teflon in it and a good low temperature viscosity rating. Others, like the hammer weight I leave dry, but highly polish anything that has "witness marks" on it from rubbing.
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It seems to me that if he increases the regulator pressure he will just magnify the lower velocity. Increasing the hammer tension would be the better way to overcome the problem.
 
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@DesertSilver. Thank you for chiming in. I might adjust the HS first to see if I can get the speed up before increasing the Reg pressure. As I mentioned before that I really didn't want to mess with what I had before when everything was running perfectly before the temperature drop. I can still hit 50 yards target confidently with the way it is now. I will just not shoot longer ranges unless I have to. And I have yet to chrony the check the groupings with the lighter 23gr slugs and see if I get good enough result for the winter months.
 
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Max115 said:
@ToddGeddes63. Good idea with shooting lighter weight pellet to get the speed up. I will have to try the 23gr slugs in my Impact to see if I can group well with the lighter weight. My gun is tuned to shoot the 25gr slugs at 984fps.

@dens228. Interesting method. Will check it out, thanks.
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Have you been able to try the lighter pellet method? I'm interested in your results.
 
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ToddGeddes63 said:
Have you been able to try the lighter pellet method? I'm interested in your results.
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Hi Todd, I did try the 21 and 23gr H&N slugs without making any adustment to the Regulator. I turned the HS only. The groupings were okay, but I still find the 25gr to be the best in my current setting. I am leaving the gun as is and adjusted the new POA for each distance.
 
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nervoustrig said:
You're on the right track regarding the regulator's operation. As the temperature falls and the pressure inside the plenum declines as a result, the regulator's valve seat will open up to top off the pressure. At least it should. There's some amount of hysteresis which will vary from regulator to regulator, and the topping charge is slow (like pressure creep). But unless the gun's state of tune is out of balance, the pressure difference should be small enough that it alone would not explain any meaningful velocity change.

Another factor at play is potentially disparate coefficients of linear thermal expansion between different materials of the regulator. For example, Delrin has a coefficient 5x greater than the metal parts of a typical regulator. So for a regulator with a piston made entirely of Delrin (e.g. some FX regulators), it shrinks at 5x the rate of the metal regulator body. The absolute difference is very small, but enough to influence the setpoint considering air molecules are in the realm of 1 - 2 angstroms.
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One thing I’m not seeing discussed here is the temperature induced change in the ability of air to compress and expand efficiently thus storing the energy to power the shot.
I’ve noticed a drastic change in power levels based on temperature. As stated, as the air cools and contracts in the plenum the reg should open and top it off. However my experience doesn’t seem to show this, I seem to see drastic reduction in velocity at the muzzle in very cold weather.
So I have the theory that colder air is denser, thus isn’t able to compress as much and not storing as much energy in the plenum to be released at the shot. Also denser air is heavier and thicker, not flowing as well through the valve, transfer port and barrel. All this combines to reduce performance in cold weather.
 
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JDGriz said:
The speed of sound inside the barrel matters to performance independently of starting pressure.
The speed of sound determines the speed of expansion of a volume of gas. The molecules can't move away from one another any faster than their individual speeds due to temperature (which is the speed of sound). This is one of the reasons air gun efficiency drops as pellet speeds approach the speed of sound at normal temperatures - like the ones inside the barrel and tank. The speed of sound is much higher in a hot gas, like the one produced by gunpowder.
So maybe it's just that: the same amount of reg pressure pushes less on the pellet on a cold day, because the gas expansion is slightly less able to catch up with the pellet as the pellet approaches the same absolute final speed in the barrel. The change in the speed of sound is less than 0.5% between 0 and 20 deg. C - 1085 fps vs 1124 fps, but the change of the difference between this and the pellet (average) final speed is larger (assuming it matters). Maybe it's like having a slightly shorter effective barrel length, on a colder day.

There are ways around this problem, if it is a problem, on the air gun design side, but I think people mostly stumble on it by chance and not necessarily by design intent:
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Interesting concept. I’ve noticed it takes a lot more air to tune a gun up close to the speed of sound, and they become more inefficient. The other factor in Airguns vs powder burners is that powder burners produce heat thus increasing the speed of sound. However Airguns RELEASE pressure causing decompression and causing a drop in the temperature and speed of sound.
 
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