Effects of Ambient Temperature -- Worth Noting

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d2admin

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This is presented for entertainment only. Your results will vary.

I spoke with a gentleman at Accurate Arms yesterday about ambient temperature effects on maximum chamber pressures and velocities. I say “ambient temperature” but am not just implying air temperature, but firearm/cartridge temperature as well.
I think his name was Ted. He didn’t have any information on chamber pressures, but had done some work with Speer back in the 70’s and hence pulled out a Speer manual (#17???) and read some numbers off for me. He didn’t tell me what the firearm/caliber was (I suspect it was a military round), but gave me this information:

0F 2630fps
70F 2722fps
100F 2827fps


Then I rummaged around the garage last night and found a “Reloading Guide for Rifles and Handguns” from Vihta Vuori Oy, noted 1-97. This was one of those freebie-types you pick up at the gunshop in amongst all the reloading supplies. This manual had a little more specific information:

For 9mm Luger, 5.56grs of N330, 115grFMJ:

-65F 14200 psi 1046fps
70F 25900 psi 1233fps
126F 27700 psi 1249fps

For .308 Win, 45grs of N135, 147grFMJ:

-67F 38000psi 2542fps
70F 50900psi 2742fps
140F 55200psi 2808fps


This manual provides a small table of Interior Ballistic Variables, one item being Powder Temperature. It notes a good quick rule of thumb is the relationship for a +50F change in powder temp, add 2% to velocity and add 4% to maximum chamber pressure.


So I plotted some of this information up to see what it looked like. I found the +2%velocity for +50F rule to be fairly representative, but not the +4% in Pmax-- the data reflects a steeper slope, more on the order of +6% to +8%.


View


View




I noted earlier reference to a table of Interior Ballistic Variables:

Bullet weight +10% ....–4% velocity……….+8%Pmax
Charge weight +10% …….. +8% velocity …….+20% Pmax
Power temp +50F ………… +2% velocity ………. +4% Pmax
Cartridge case volume +10% ………..-7% velocity ……… -40% Pmax


While changes due to ambient temperature don’t appear as extreme as the other variables listed, these other variables are controlled at the loading bench. Weather conditions, obviously, are not controlled, BUT… if you have worked up to a maximum load under mild weather conditions, leaving ammunition in the sun during a hot summer day could create overpressure problems.
Worth noting.
 
In P.O. Ackley's book " Handbook For Shooters and Reloaders" Les Bowman wrote a chapter on the effects of temperature on velocity and pressure in which Dupont states that they figure 60 psi in pressure difference for every degree in powder temperature change. You're right, Encore, that powder temperature is the deciding factor and not ambient temperature.
 
So from this information, I have been developing loads recently at around 40°F for a trip to Canada where I am expecting - 5 to 20°F I should have lower velocity, pressure and more curved trajectory, or is the differnce not enough to be discrenable?
Another thing I was wondering about is lubrification. Will narmal gun oils freeze up that those temperatures?
Thanks
 
This is presented for entertainment only. Your results will vary.

I spoke with a gentleman at Accurate Arms yesterday about ambient temperature effects on maximum chamber pressures and velocities. I say “ambient temperature” but am not just implying air temperature, but firearm/cartridge temperature as well.
I think his name was Ted. He didn’t have any information on chamber pressures, but had done some work with Speer back in the 70’s and hence pulled out a Speer manual (#17???) and read some numbers off for me. He didn’t tell me what the firearm/caliber was (I suspect it was a military round), but gave me this information:

0F 2630fps
70F 2722fps
100F 2827fps

Then I rummaged around the garage last night and found a “Reloading Guide for Rifles and Handguns” from Vihta Vuori Oy, noted 1-97. This was one of those freebie-types you pick up at the gunshop in amongst all the reloading supplies. This manual had a little more specific information:

For 9mm Luger, 5.56grs of N330, 115grFMJ:

-65F 14200 psi 1046fps
70F 25900 psi 1233fps
126F 27700 psi 1249fps

For .308 Win, 45grs of N135, 147grFMJ:

-67F 38000psi 2542fps
70F 50900psi 2742fps
140F 55200psi 2808fps

This manual provides a small table of Interior Ballistic Variables, one item being Powder Temperature. It notes a good quick rule of thumb is the relationship for a +50F change in powder temp, add 2% to velocity and add 4% to maximum chamber pressure.


So I plotted some of this information up to see what it looked like. I found the +2%velocity for +50F rule to be fairly representative, but not the +4% in Pmax-- the data reflects a steeper slope, more on the order of +6% to +8%.

fca64023.jpg


fca64022.jpg






I noted earlier reference to a table of Interior Ballistic Variables:

Bullet weight +10% ……. –4% velocity……….+8%Pmax
Charge weight +10% …….. +8% velocity …….+20% Pmax
Power temp +50F ………… +2% velocity ………. +4% Pmax
Cartridge case volume +10% ………..-7% velocity ……… -40% Pmax

While changes due to ambient temperature don’t appear as extreme as the other variables listed, these other variables are controlled at the loading bench. Weather conditions, obviously, are not controlled, BUT… if you have worked up to a maximum load under mild weather conditions, leaving ammunition in the sun during a hot summer day could create overpressure problems.
Worth noting.
 
That's pretty much what I asked on another thread yesterday. Great info. Thanks for sharing. Aside from the obvious safety issues, how much would that affect accuracy? If the barrel vibrations change with the pressure and velocity changes, in theory, you may not even be close, correct? Or are these differences negligible. Seems like a lot to me. Or am I way outta there?
Great Post. Thanks
GJ
 
it appears that temperature changes can affect group SIZE as well as point of impact. while making non-scientific experiments with a browning rifle equipped with a BOSS, i usually found a couple of "sweet spots" where groups would narrow down surprisingly small, as per browning advertising. however, when i just happened to shoot the same ammo and the same BOSS setting at a different time of year, when the ambient temperature was a lot different, the groups were NOT nice and tight. i was sure disappointed, until i realized that, perhaps for a variety of reasons, barrel vibrations change with temperature.

it seemed impractical to me to re-experiment with the BOSS throughout the year. (it takes a lot of time to find a sweet spot, allowing the barrel to cool after every 3 shots or so). besides, the original 'muzzle brake' BOSS increased noise levels noticeably, so i sold that rifle.

now, the simplest and least time-consuming procedure for me is to just shoot at the range in various temperatures throughout the year, and record the varying accuracy and point of impact results. kinda like using "kentucky windage."

if you can do it, the idea mentioned earlier about doing your ammo and sight-in work at a temperature similar to what you expect to hunt in, is clearly a great idea.
 
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