Temperature inside/outside pellet velocity change

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ladwict

ladwict

Member
Aug 10, 2020
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Holland, Netherlands
I was taking my wildcat out this morning at around freezing temperature. Within 2 minutes I noticed my pellets speed was lower than with higher outside temps . Is it possible that within minutes your shot velocity shows velocity drop down by the outside temp ? Is it the air in the airtube that cools down that fast and causes the velocity loss or is the air coming through the venting hole or even any other reason ? 
Please let me have your experiences/opinions on what I noticed this morning . 
 
I have to add 1 MOA of elevation to my 40 yard zero in the winter. Bantam Sniper HR, JSB Exact Jumbo 18 at approximately 875 fps. I typically leave my gun outside for at least fifteen minutes in the cold then shoot for groups.
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Good advice here. I don’t leave mine outside that long before shooting, but after filling inside 73-76 F and knowing that it’s below 65F outside I am in the habit of setting my rifle outside for about 5 mins before shooting. If I’m hunting in the 35-45F weather I’ll leave it outside longer before embarking. The reservoir usually loses a little pressure in cooler weather. I wish I paid closer attention to say how much pressure is lost. Anyone think the loss in pressure (in cooler outside ambient temps) is the culprit for lower velocities in unregulated guns or is the pressure loss negligible?
 
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Anyone think the loss in pressure (in cooler outside ambient temps) is the culprit for lower velocities in unregulated guns or is the pressure loss negligible?
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Going from 70F to 30F will drop a 3000psi reservoir to a little below 2800psi.

For any unregulated PC with a 3000psi max fill tuned for a useful bell curve, the drop in pressure would produce a velocity increase because it would be on the upward part of the bell curve.  That is, assuming pressure is the only influential variable that changed with the temperature. But of course it isn’t. The colder atmosphere is more dense, lubricants become more viscous, materials contract at different rates, etc.
 
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Is it possible that I see velocity drop within minutes from warm shooting indoors to cold shooting outdoors ( 12 fps lower) ?
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Yes, whether unregulated or regulated (l

Yes, whether unregulated or regulated (like your Wildcat), it is certainly possible to have a small velocity loss after just a few minutes. 

Indeed, everything will not have reached thermal equilibrium in 5 minutes. Does it continue to decline past the first few minutes? What about 10 minutes, 15 minutes, 20 minutes? If so, I would say that reasonably confirms it.
 
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Is it possible that I see velocity drop within minutes from warm shooting indoors to cold shooting outdoors ( 12 fps lower) ?
Click to expand...

Yes, whether unregulated or regulated (l

Yes, whether unregulated or regulated (like your Wildcat), it is certainly possible to have a small velocity loss after just a few minutes. 

Indeed, everything will not have reached thermal equilibrium in 5 minutes. Does it continue to decline past the first few minutes? What about 10 minutes, 15 minutes, 20 minutes? If so, I would say that reasonably confirms it.
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Thanks . Good info. I notice the velocity drop from 265fps indoors to 255 fps when I take the gun outdoors in the cold and start shooting after a few minutes. Temp drop is from 77 to 34 degrees Fahrenheit. I never tried keeping the rifle out and try again after 20 minutes. What should I expect ?
 
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What should I expect ?
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It depends on too many variables to predict with any degree of certainty. For example, a close tolerance hammer and tube with a little oil or grease to lubricate it...it could lose over 100fps in the cold. Learned at the school of hard knocks. 

My recommendation to chrony it at 5, 10, 15, 20 minutes was so you can characterize the extent to which your particular rifle is sensitive to temperature. That’s the only real way to know.
 
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What should I expect ?
Click to expand...

It depends on too many variables to predict with any degree of certainty. For example, a close tolerance hammer and tube with a little oil or grease to lubricate it...it could lose over 100fps in the cold. Learned at the school of hard knocks. 

My recommendation to chrony it at 5, 10, 15, 20 minutes was so you can characterize the extent to which your particular rifle is sensitive to temperature. That’s the only real way to know.
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Thanks again. Will try !
 
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Pretty simple to find out if temp is culprit (which it most likely is) by reversing the process. Fill the rifle outdoors in the cold , then take in to warm environment. Pressure will rise as air heats up. The more variation in temps will make difference more noticeable. This change will happen pretty quick if tank is steel of aluminum. You could even return the gun to cold and pressure will go back to cold fill pressure. 10 degrees will probably not have a big impact, but 30 degrees would be noticeable. I would think there is a formula for calculating, but the best would be to prep in the environment you're shooting in. Unregulated, you could probably fill in cold, say the fridge, and shoot in warm to keep pressure more level. Ok, getting complicated, forget all that and go shoot. Good evening to all. 
 
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I live in the south so I will not be able to do this experiment myself.

I am no scientist, but I think that cooling the barrel changes the bore a few 10 thousandths of an inch. I understand the effects of cold lube but I also believe the tiny amount of bore shrinkage is causing a tighter fitting pellet. If you size your pellets, try the undersize ones in the chilled barrel and let us know the results.
 
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Bore friction is pretty insignificant with pellets so it would take a dramatic differential thermal expansion rate to make much of a difference in velocity. And for the scenario we're discussing where things are cooling down, the pellets would shrink more than the bore, very slightly alleviating friction (lead's coefficient is about 2.5x that of steel). So if anything, this effect might produce a subtle increase in velocity, not the decrease we usually see.
 
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I have made some new tests. I have taken the Wildcat that is shooting 965 fps at 70 degrees Fahrenheit outside and left the rifle there for half an hour at 32 degrees Fahrenheit and the average speed decreased with approx. 26 fps. Enough to change grouping pattern ( in this case for the worse) . The regulator was outside nicely getting back to the same pressure as when shhoting at 70 degrees inside. So my conclusion is that there must be a slower airflow or more friction in this lower temp. My hammer (spring) is set up completely dry so no viscosity changes that could affect speed I guess. Your thoughts ? 
 
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