I was surprised when my most accurate PCP (my P35-22) shot a couple sub-190 30 yard challenge targets. I think the secret is the barrel was dirty but in my investigations I was also surprised to find the velocity was about 40 fps slower than in the summer. About 5% lower. I looked up the specific heat difference and from 85 F to freezing it was only about 1% lower. Some may be lubricant on the hammer thickening up but I also wonder if some is not the little bit of water in the compressed air. It's really dry here now and definitely not at all dry in the summer. I don't think I am at 100% relative humidity at 300 bar even in the summer. I see no evidence of condensation inside the air chamber of the gun. But it seems logical that there may still be a little more water in the compressed air in the summer. And less in the winter. Could the lower specific heat of the compressed air be a large part of the reason our airguns shoot slower in the winter?
Is my airgun 5% slower in the winter due to the specific heat of the air?
- Thread starter JimD
- Start date
I had setup an airgun with an acetal hammer. When first using it at higher temperatures, velocity fell off a lot. The problem was the higher coefficient of thermal expansion of the acetal reduced the clearance in the steel tube to the point where the hammer was not moving freely enough. I took a few thousandth off the hammer diameter and it was good.
If the P35 has a steel hammer inside an aluminum receiver and the clearances are tight, you could be having more hammer drag as the temperature falls. At reduced temperatures, aluminum shrinks considerably faster than the steel.
If the P35 has a steel hammer inside an aluminum receiver and the clearances are tight, you could be having more hammer drag as the temperature falls. At reduced temperatures, aluminum shrinks considerably faster than the steel.
Upvote 0
H2O has a lower molecular weight than most of the different gases that make up the atmosphere, thus
air density is inversely proportional to relative humidity.
Air density is inversely proportional to air temperature.
Air density is proportional to atmospheric pressure.
A measure that correlates temperature and relative humidity and atmospheric pressure is Density Altitude. Check your closest FBO with a weather station. If the DA is negative the air density is high and your pellet will be pushing against more molecules per length of travel, if the DA is positive your pellet will have fewer molecules to shove out of its way.
At this moment the conditions at MDT are 33°F, 64 % Relative Humidity, 30.03 in. Hg atmospheric pressure, yielding a density altitude of -1,480 ft.
Which is some dense air to shove a pellet through.
air density is inversely proportional to relative humidity.
Air density is inversely proportional to air temperature.
Air density is proportional to atmospheric pressure.
A measure that correlates temperature and relative humidity and atmospheric pressure is Density Altitude. Check your closest FBO with a weather station. If the DA is negative the air density is high and your pellet will be pushing against more molecules per length of travel, if the DA is positive your pellet will have fewer molecules to shove out of its way.
At this moment the conditions at MDT are 33°F, 64 % Relative Humidity, 30.03 in. Hg atmospheric pressure, yielding a density altitude of -1,480 ft.
Which is some dense air to shove a pellet through.
Upvote 0
As well as the explanations in the previous posts, any oil or grease within the gun will become more viscous, even if present in small quantities it will impede the movement of the guns internals,
Bb
Bb
Upvote 0
In the fall last year i shot my Maverick 2 times, but the kicker is in between the two times the temperature had dropped from 15 deg C to like 5 deg C
My Maverick did NOT like that at all, where the 13 gr slugs was flying really good on my previous outing in the last one it was so bad i pulled the Heavy liner and put in the regular one and shot some pellets at shorter distances.
My Maverick did NOT like that at all, where the 13 gr slugs was flying really good on my previous outing in the last one it was so bad i pulled the Heavy liner and put in the regular one and shot some pellets at shorter distances.
Upvote 0
I shot my P35-22 some more today trying different hammer spring settings to see how much velocity I could regain. I ran out of hammer spring before reaching the point where velocity stopped increasing. I was a little over 2.25 turns higher than my summer tune. I gained back less than half the velocity loss due to temperature. Maybe if I could turn the hammer spring up more I could get back half the loss. So regardless of whether it is tighter clearances between the steel hammer and the aluminum tube it is in or lubrication on the hammer thickening a good bit of the velocity loss seems to be the hammer isn't hitting the valve as hard. Probably about 1/2.
The other roughly half the loss seems likely to be reduced energy in the compressed air - regardless of how you describe that.
The other roughly half the loss seems likely to be reduced energy in the compressed air - regardless of how you describe that.
Upvote 0
"Some may be lubricant on the hammer thickening up"
I only use a DRY lubricate on my hammer , I have both dry moly and HbN
I only use a DRY lubricate on my hammer , I have both dry moly and HbN
Upvote 0
While shooting over the chronograph, if you had access to power for a blow dryer or heat gun, you could warm the action up some and confirm or discount some of the theories as to why.
Upvote 0
Exactly what I was thinking earlier. Shoot the rifle indoors at room temp through the chrono. The take the rifle outdoors and let it get to outdoor temp for about two hours, then shoot through chrono. If that doesn’t work I have no idea how to approach the issue.
Upvote 0
Colder drier weather will slow everything. As stated above, density altitude increases as temperature and humidity drop. An airplane will climb better and use less runway on a cold dry day. I’m not surprised that the denser air slows the pellet speed and colder temperatures decrease air pressure.
Rick H.
Rick H.
Upvote 0
IMO atmospheric density changes couldn't be drastic enough to make measurable changes of velocity at or near the muzzle in a PCP. If the power is down at the muzzle its because of something else.
I also believe that compressed air at a given pressure has the same amount of potential energy regardless of ambient temperatures. Meaning PCPs at a given tank pressure shouldn't suffer power changes from ambient temperature or even atmospheric pressure change. Basically 300 bar is 300 bar, hot or cold. Temperature and atmospheric pressure might change how long it takes to make 300 bar but once there its potential is the same.
Springers on the other hand can only pressurize a limited amount of available air and thus more subject to environmental conditions. My springers actually make a little more power in cooler denser air. Either way temperature certainly can affect the mechanical parts of both types of guns. If your PCP gun is affected by temperature it's likely a mechanical issue involving clearances and or viscosity. There's been some good suggestions here about how to test that.
I also believe that compressed air at a given pressure has the same amount of potential energy regardless of ambient temperatures. Meaning PCPs at a given tank pressure shouldn't suffer power changes from ambient temperature or even atmospheric pressure change. Basically 300 bar is 300 bar, hot or cold. Temperature and atmospheric pressure might change how long it takes to make 300 bar but once there its potential is the same.
Springers on the other hand can only pressurize a limited amount of available air and thus more subject to environmental conditions. My springers actually make a little more power in cooler denser air. Either way temperature certainly can affect the mechanical parts of both types of guns. If your PCP gun is affected by temperature it's likely a mechanical issue involving clearances and or viscosity. There's been some good suggestions here about how to test that.
Upvote 0
300 bar tank pressure at room temp (65°f) will decrease as the tank cools down to outside temp (20°f).
Pressure in the tank drops as it cools because molecular motion decreases as temperature decreases, increasing air density which increases head loss.
A decrease in differential pressure across the poppet valve combined with the increase in head loss reduces the flow rate of air to the pellet per unit time which reduces the pressure increase per unit of time reducing the kinetic energy delivered to the pellet per unit time.
Less kinetic energy to the pellet means lower velocity at the muzzle which coupled with a low density altitude changes the flight time of the pellet and POI.
Combine all this with the friction changes in moving mechanical components of the gun and the result will be varying performance over varying temperatures.
To reduce the change in performance at low temperatures: charge the tank to 300 bar at the temperature you will be using the gun and reduce the restriction of air flow to the pellet.
Last edited:
Upvote 0
All the above makes me really appreciate my compound bow when deer hunting… regardless of humidity, rain, hot, cold, very cold, in the 20 to 25 yards that I’ve harvested my deer, none of that has to be compensated for. 
Upvote 0
Yes we're saying the same thing. I understand 300 bar inside will change as you move to hot or cold outside. I didn't say that it didn't. I said bringing it to 300 bar changes with atmospheric conditions. Once at 300 bar at either condition, 300 bar is 300 bar. Again moving to different temperatures and atmospheric pressure does change the pressure in the tank which changes everything. That's obvious. What I'm saying is 300 bar is 300 when stabilized at 300 in any temp or atmosphere. If you move a 300 bar tank to cold temperatures the pressure will drop and is no longer 300 bar. Arbitrarily say reduces to 290 bar. It can't give the same performance you'll have to pump it back up to 300 in the cold. Likewise pressure increases as your atmospheric pressure decreases around the tank. Your tank would gain pressure simply by going from NY to Denver. My friend back in 80s air shipped his $5k triathlon bike to Hawaii for the Ironman. He made the mistake of shipping it with the tires inflated. When he unpacked the bike the tires had already exploded and destroyed thousands of dollars of carbon wheels. It was either shipped in an unpressurised area of the aircraft one that had faulty pressurization equipment. I can't imagine how much pressure the tubular tires reached before they exploded. Their normal pressure IIRC was ~190psi at sea level where he lived. Lord only what that reached at 35,000 feet.
Yes air density changes the resistance against a projectile over the length of the flight and understandably can change velocity and poi at the target. Most airgunners don't shoot far enough for it to be noticeable among other larger variables. I imagine at the longer ranges a PCP is capable it can be a factor. Particularly if shooting pellets which have terrible BCs to begin with.
I think we're pretty much saying the same thing. I was probably unclear in my initial explanation.
Upvote 0
I'm headed to the store to get some dry lube. I want to see if I can use some brake cleaner to take any oil off the hammer and inside of the tube and put a little powdered graphite on and measure a slight increase in velocity. I might try using a heat gun, I have one, but I would want to be careful with it.
Specific heat is a measure of energy in air. If you google "specific heat of air at different temperatures" you can find the things I am looking at. The engineering toolbox is one source I've consulted. It lists the Isochoric (constant volume) specific heat of air at 32 F as .02077 KJ/mol and the value at 80 F is .02080. That is only about a .1% change, however. I must have done the math wrong last time when I estimated 1%. But the fact remains it isn't enough to cover the change in velocity - basically it's trivial. Water vapor has about 3 times the specific heat as air but we have to speculate at the relative humidity change to see what the energy change would be in the compressed air. But I don't think this will explain anything close to 5%. But my hammer spring changes demonstrate the reduction in hammer spring force is a major part of the difference. If that can be "fixed" with dry lube that may be enough.
I got more interested in this after shooting several 30 yard challenge targets that were under 190 with my most accurate airgun (my P35-22). Last night I noticed a loose scope mount, however, and this morning I shot a 197 with it. So the gun is still accurate with a little less velocity but it would be nice to loose less velocity. A change in the energy in the air (or the density of the cold air) isn't something I can fix, except possibly for a few minutes in a test.
Specific heat is a measure of energy in air. If you google "specific heat of air at different temperatures" you can find the things I am looking at. The engineering toolbox is one source I've consulted. It lists the Isochoric (constant volume) specific heat of air at 32 F as .02077 KJ/mol and the value at 80 F is .02080. That is only about a .1% change, however. I must have done the math wrong last time when I estimated 1%. But the fact remains it isn't enough to cover the change in velocity - basically it's trivial. Water vapor has about 3 times the specific heat as air but we have to speculate at the relative humidity change to see what the energy change would be in the compressed air. But I don't think this will explain anything close to 5%. But my hammer spring changes demonstrate the reduction in hammer spring force is a major part of the difference. If that can be "fixed" with dry lube that may be enough.
I got more interested in this after shooting several 30 yard challenge targets that were under 190 with my most accurate airgun (my P35-22). Last night I noticed a loose scope mount, however, and this morning I shot a 197 with it. So the gun is still accurate with a little less velocity but it would be nice to loose less velocity. A change in the energy in the air (or the density of the cold air) isn't something I can fix, except possibly for a few minutes in a test.
Upvote 0
Since it’s a regulated airgun, plenum pressure while shooting will be relatively consistent, even with fairly wide pressure variation in the tank
Upvote 0
I agree, I don't see why the regulator would change due to temperature so I think the regulated pressure is not a source of the velocity decrease.
I took the hammer out, cleaned it with brake cleaner (it has an oily lubricant on it) and then tested it with nearly nothing on it and then again with a dry lube on it. The velocity didn't change a lot. I measured three shots in each case. Before I started making changes I measured 776.9, 789.3, NS 791.6 fps. With the old lubricant cleaned off (as best I could) I measured 782.7, 788.6, and 787.1 fps. Last I measured velocity after application of graphite based dry lube at 791.6, 790.8 and 796.8 fps. So a little higher with the dry lube perhaps but not much different.
The temperature during these tests was 63 degrees F. Not very cold for the winter time even in South Carolina.
I measured the velocity with the same tune several times at 835 fps last summer. So still I am about 5% low. I've thought of a reason other than temperature, however. One of the grub screws that holds the cocking rod in place fell off at some point last year and fell down into the hammer area. I shot the gun a few times with it in there. That undoubtedly scratched the aluminum tube some and could be a source of a decrease in the hammer force. I'm not sure I checked the velocity after that point. I initially thought the decrease was temperature but maybe it's not that simple.
I took the hammer out, cleaned it with brake cleaner (it has an oily lubricant on it) and then tested it with nearly nothing on it and then again with a dry lube on it. The velocity didn't change a lot. I measured three shots in each case. Before I started making changes I measured 776.9, 789.3, NS 791.6 fps. With the old lubricant cleaned off (as best I could) I measured 782.7, 788.6, and 787.1 fps. Last I measured velocity after application of graphite based dry lube at 791.6, 790.8 and 796.8 fps. So a little higher with the dry lube perhaps but not much different.
The temperature during these tests was 63 degrees F. Not very cold for the winter time even in South Carolina.
I measured the velocity with the same tune several times at 835 fps last summer. So still I am about 5% low. I've thought of a reason other than temperature, however. One of the grub screws that holds the cocking rod in place fell off at some point last year and fell down into the hammer area. I shot the gun a few times with it in there. That undoubtedly scratched the aluminum tube some and could be a source of a decrease in the hammer force. I'm not sure I checked the velocity after that point. I initially thought the decrease was temperature but maybe it's not that simple.
Upvote 0