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Old April 30, 2007, 07:56 PM   #1
Ammo Junky
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fps change with temp change?

I tested a 308 load @ 55F and got 2683
Tested @ 84F I got 2766
Thats a change of 83 fps for 29F
Is this about normal.
About 3fps / 1*F

150 rem core lock, rem x1 fired fl sized brass. WLR primer, 43.0gr H4895 2.790" col

Rem 700 vs 26" bbl
Will work for brass.

I apologise in advance for spelling errors.
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Old April 30, 2007, 08:12 PM   #2
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It's called air density.
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Old April 30, 2007, 09:21 PM   #3
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My reloading mentor once wrote an article about loads he used while test driving an Oheler chronograph. In the article, mentioned the fps difference in .223 using H335 powder. SO...might be a pressure increase also, nice to know if your running at max loads, or close to it!
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Old April 30, 2007, 09:24 PM   #4
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I think its possible to see a very slight rise in FPS due to hot conditions. I could see the barrel geting hot and expanding, there by generating less friction as it travels down the barrel. resulting in greater FPS, total speculatin, ive got no proof.
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Old April 30, 2007, 11:26 PM   #5
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H4895 is fairly temperature "sensitive". This is the reasoning behind several of the "newer" powders. Hodgdon "extreme" powders, some of the Vihtavuori powders, etc.

I load .308 Win with Varget (Hodgdon Extreme powder) and have found the speed/pressure increases are substantially less.

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Old May 1, 2007, 11:52 AM   #6
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Air density doesn't have time to have an effect when the chrono is set within a few yards of the muzzle.

Some powders are temperature sensitive, and some are more stable. A variance of 1fps/1*F is not uncommon.

Here is an experiment

In my own experiment, I have seen as low as 10-15 fps difference for 90F temperature difference using Varget and H4350, but it may be load-dependent.
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Old May 1, 2007, 12:19 PM   #7
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Old May 1, 2007, 01:35 PM   #8
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I have done a lot of shooting in temperatures form 10 to 100 degrees F. Velocity variations are noticeable, especially with the older powders varieties. However, cold weather shooting, temperature for –10 to –40 F, is another world. Powders advertised to perform consistently over a wide range of temperatures, (i.e. freezing to 100 F), perform differently in subzero weather in my experiences.

On more occasions than I care to count, I have hunted in subzero weather. One memorable hunt went 3 days at –44 to –38 degrees F. I have shot many game animals at temperature of -10 to -30 F, and have definitely noticed reduced velocity and reduced terminal performance of the bullets. Although I have a chronograph, I have never shot loads over it in subzero weather, because the wind was usually blowing at too much velocity. I also do not have a propensity for self-abuse in order to sit and shoot at low temps.

I base the following opinions, not on pure scientific data, but on the pseudo science of my personal field observations and comparisons of the wound channels in lots of game taken at very low vs. moderate temps, using a variety of cartridges.

Once you get well below zero, there are other factors at work on velocity besides powder. It is a physical fact that metal shrinks in cold temperatures (consider the old trick of removing stuck cases from loading dies by putting them in the freezer over night before extraction). Barrels are slightly constricted and probably impede bullet speed. Bullet jacket alloys must perform differently as well, their ability to deform and glide down the barrels is probably changed. Besides physical factors, I believe there are a number of chemical factors at work in sub zero temps: primer ignition is lessened, powders have reduced performance, whether from reduced burn rates, temp of burn, temp vs pressure reactions, and so on, as I am sure there are others.

I have always tried to work up loads at the bench in the weather in which I plan to hunt. I have never been able to work up loads much below freezing. Warm weather loads fired in cool temps are one thing, but shooting cold weather coyote loads at prairie poodles in 100 degree F weather could be suicidal. You have to make sure the two extreme loads are not mixed.
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Old May 1, 2007, 03:04 PM   #9
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The 2007 Hornady manual addresses this subject. Barometric pressure will affect muzzle velocity. But even in an extreme instance it is only about 50-60 fps.
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Old May 1, 2007, 04:23 PM   #10
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The decomposition reaction of smokless powder speeds up with temperature.
Hotter powder, increased burn rate.
Various deterents are used to try and minimize this in a number of powders, but if you get almost any powder warm enough it burns faster.
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Old May 1, 2007, 04:54 PM   #11
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H4895 is fairly temperature "sensitive
Yes I wondered about that, I have mostly used varget and 165GK in the past. I started with the H4895 and the 150 core locks just to use then up.

I could see the barrel geting hot and expanding, there by generating less friction
The bbl was hotter than I like, because of circumstances sunny, temp etc. It was never too hot to cary the rifle by the bbl (no I dont carry it that way), but hotter than I like. Bbl friction is a funny thing. More friction can increase mv becaus of increased pressuar and increasec powder speed . I dont know if there is a point that reducing friction could cause an increase of mv above normal or not.
Will work for brass.

I apologise in advance for spelling errors.
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Old May 1, 2007, 08:21 PM   #12
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Old rule of thumb

You get less performance (from almost everything) in cold to very cold weather. Manufactures try hard to improve it, and many products today are better than they used to be, but it still remains generally true.

The old rule of thumb is to always work up your loads in hot weather. If a load is safe in hot weather, it will be safe in cold weather. The reverse is not necessarily true.

And even in hot weather, some common sense precautions are in order, such as don't leave your ammo in direct sunlight for long periods, or worse yet, in a car window.

Expect a velocity loss when shooting in cold (below freezing) and very cold (below 0) temps. Some powders are more sensitive about this than others, but all are affected to some degree.
All else being equal (and it almost never is) bigger bullets tend to work better.
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