Cold temperature affecting speed of projectiles

Jump to latest Follow Reply
Max115

Max115

Member
Jul 15, 2018
1,773
1,428
BC, Canada
Well, I finally got out to do some plinking with my FX Impact MK2 at the farm after almost two weeks of raining in my region.

It was a beautiful sunny day with some wind, nothing I haven't seen before. I even re-did my range tape on my turret where I had marked out 50-150 yards including the 240 yards of potato shooting I did last month (video to follow). So I set up my target box at 50 yards to double check my zero. first 3 shots were missing to the left and low. I thought, okay, it was the wind and my gun was warming up. Took a few more shots and I was getting shotgun patterns. WTF.

I had this rifle dialled in almost perfect since the last time I shot at the end of Oct. It was shooting the H&N 25gr Gen2 slugs at 984 fps and I was dropping starlings and doves out past 100 yards with ease. Now it was spraying out of control. So I took out my FX Chrony and got different velocity reading of from 951 - 979 fps with an average of 969 fps. It was 15 fps slower than before and that had changed the accuracy of my previously tuned gun. I tested out my accuracy out to 100 yards and all my range tape marking were incorrect and I wasn't able to consistently hitting my 3/4" bullseye.

What a bummer. Now I have might have to increase my Reg pressure up and adjust the hammer spring to reach 984fps again with the 25gr slugs. Luckily I know what the optimal velocity my gun needs to be with the 25gr slugs, so that shouldn't be an issue. However, putting the time and wasting slugs again is a pain.

I will have to make the adjustment next time out, but for the meantime, I will keep it as is as I am planning to get coyote#8 hunt in the next few days. Farmer told me the coyote(s) had returned in the last couple of days and dug up the two still born calves that he buried in the pit where I shot coyote#7 back in June.

I setup the trail cam today and I will review the footages in the next two days and see what the activity entails. Fingers crossed that I will make my encounter when I go for the hunt.
 
  • Like
Reactions: .20calguy, Sqwirl57 and Ctkny
@Max115 Here's a tuning trick that may work for you... get the tune/speed back with a higher reg pressure but a slightly lower hammer spring tension dial number. (Leaving some dial tension in reserve.) This way when it's time to hunt or shoot when it's is colder outside you can simply add a click or two of additional hammer tension to get back the optimum velocity.

While I like the Impact I bought back in the Gen1 period, and tinkered and upgraded things through the Gen2 period, the constant chasing of the scope's zero becomes old after a while. Every time I took it out in the elements the different hot and cold cycles would change it's characteristics and point of impact slightly.
 
  • Like
Reactions: .20calguy
Upvote 0
.20calguy said:
I don't have all that going on, but my springer an HW30 was shooting low after the temps dropped. It was noticeable because I have been shooting it at 75 yards. I've been busy, and haven't had a chance to addess it yet.
Click to expand...
My springers shooting low too. But only for the first few shots until the mechanism warms up.
 
  • Like
Reactions: .20calguy
Upvote 0
@.20calguy. Yes, I had the same issue back in the days when I was shooting my HW97. The colder the temperature, the lower the power output.

@Airgun-hobbyist. Thank you for the tips. I probably wouldn't make any adjustment just yet. I want to focus on the coyote hunt for now. I might do a video next time I am out at the farm to compare the accuracy between the 25gr and 23gr slugs. To see if the lighter slugs would get faster speed in the 950 fps range and if the grouping is tighter, then I will use the 23gr for the winter time. Yes, lots of tinkering but the Impact has the capability to do just that. I couldn't do any adjustment with my Daystate Regal; luckily it is accurate with the right pellets.

@Septicdeath. Ya, it is common. Cold temperature decreases the speed and vice versa.
 
  • Like
Reactions: .20calguy and Airgun-hobbyist
Upvote 0
With my Cayden when My poi is low due to cold weather, I use the appropriate holder under mark and shoot three to see if the pellets are grouping, then make adjustments to the transfer port. I will also go to a lighter pellet to offset the decrease in energy. Relative to the cold weather part of the topic. Filling your AG in the cold is fine while you are outside shooting. But don't fill it to the max outside, and then take it inside a warm environment. The air will expand and could blow a burst disk or damage Orings.
 
  • Like
Reactions: wolfmangreg and Airgun-hobbyist
Upvote 0
When I shoot in the cold, mainly for squirrels and rabbits, I find that my POI is pretty much unchanged as long as I don't cock/charge the rifle until I'm on target.......that way the pressure of the loaded shot is based on PSI, not what was already in there and changes as the temp of the air in the valve drops causing less PSI. This works for 40 yards and under anyway.
 
Upvote 0
Last weekend i came out with a fairly decent setup from 3-4 weeks ago when it was 2 - 3 X warmer, but my speeds was still the same.

BUT ! the accuracy was all gone, actually to just get something in the same ballpark i had to go up 100 FPS. But alas there was no shooting 1/2" groups at 82 M, hell i was barely able to get to 1" and i was still chasing the POI around with clicking, something i did not have to do 3-4 weeks ago and to me indicative of a not good setup.
 
Upvote 0
@Peashooter. Frustrating wasn't it? To loose your accuracy after many hours/days/months of fine tuning the gun. Up until last month, my Impact MK2 was hitting almost everything out to 100 yards with ease as long as I held the proper windage. The H&N 25gr Gen2 slugs were flying true with 984 fps.

I am reluctant to make any adjustments to my REG and HS because I think I had reached the "harmonic" setup as I had before. I will give the 23gr slugs a try to see with if they group well with 2gr lighter at my current setup in this weather. My Impact still shoots okay with the 25gr but not as tight group as before.
 
Upvote 0
Privateer said:
I don't see any mention of Air Density nor the temp differences.
Click to expand...

Privateer said:
I've noticed up to 10 psi decrease in the pressure @ 30- 35 degrees and a poi change of 1/2 to a mil low. (That's the coldest temps I have shot in) Until the temp goes up to around 45-50 degrees.
Click to expand...

20211231_115926.jpg
 
  • Like
Reactions: qball
Upvote 0
dens228 said:
When I shoot in the cold, mainly for squirrels and rabbits, I find that my POI is pretty much unchanged as long as I don't cock/charge the rifle until I'm on target.......that way the pressure of the loaded shot is based on PSI, not what was already in there and changes as the temp of the air in the valve drops causing less PSI. This works for 40 yards and under anyway.
Click to expand...
While I'm not disagreeing with your experience, this doesn't make any sense. The air pushing on the valve will be the same whether the rifle is cocked or not after everything adjusts to the cold.

One thing I've noted is that reg pressure can change with the cold and will impact the projectile speed. More so based on the change in temperature. Going from 68-70 to an outdoor temp in the 40 (or the 90s) seems to have an impact on my regulated guns. So there's one benefit of an unregulated airgun in more extreme temperatures. Of course there is always possible scope shift based on temp too.
 
Upvote 0
ctshooter said:
While I'm not disagreeing with your experience, this doesn't make any sense. The air pushing on the valve will be the same whether the rifle is cocked or not after everything adjusts to the cold.

One thing I've noted is that reg pressure can change with the cold and will impact the projectile speed. More so based on the change in temperature. Going from 68-70 to an outdoor temp in the 40 (or the 90s) seems to have an impact on my regulated guns. So there's one benefit of an unregulated airgun in more extreme temperatures. Of course there is always possible scope shift based on temp too.
Click to expand...
I'm probably wording it wrong. And also admit I'm no expert on the inner workings of a PCP. If you don't cock/charge the rifle it doesn't have air to push the pellet. As the air cools the pressure drops..........my thinking is that if the pressure has reached whatever it's going to be at that temp wouldn't the regs then load the rifle up for the shot with the air pressure as dictated by the regulator? Not the volume? This does of course take variations within the function of the regulator due to temp changes.

Once again I could be way off on my thought process.
 
Upvote 0
dens228 said:
As the air cools the pressure drops..........my thinking is that if the pressure has reached whatever it's going to be at that temp wouldn't the regs then load the rifle up for the shot with the air pressure as dictated by the regulator?
Click to expand...

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.
 
Upvote 0
ctshooter said:
Going from 68-70 to an outdoor temp in the 40 (or the 90s) seems to have an impact on my regulated guns. So there's one benefit of an unregulated airgun in more extreme temperatures.
Click to expand...
In some sense, an unregulated PCP is only as good as its bell curve...meaning by how well it is tuned, how well-balanced is its lift and dwell to produce a stable velocity over a wide range of pressure. Therefore a lot more effort goes into achieving this degree of pressure insensitivity. In general, we simply don't apply this same level of effort toward tuning a regulated PCP, so we are left with the impression that a regulated PCP is more sensitive to little variations in pressure, or hammer strike, or some other factor.
 
Upvote 0
I will admit I have not read the entire thread so I may be sticking my foot in my mouth here but has anyone considered the temperatures effect on lets say a lite layer of silicon lube or other lubrication in extreme cold temps ? I am under the impression that they thicken and change in cold environments . I have experienced this in the past myself first hand . I would bet that nervoustrig would probably know the numbers at which this occurs if it is a thing haha . Just trying to help out a little or embarrass myself which ever comes first but it never hurts to learn from others knowledge is the way I take it :LOL:
 
  • Like
Reactions: DesertSilver
Upvote 0
You must log in or register to reply here.
Top Bottom
Back
Home
Forums
Resources
Menu