Temperature, Cartridge Pressure, Accuracy — The Effects of Temperature

By Dave Dolbee published on in Competitive Shooting, Safety and Training

For many shooters, reloading is a fun, calming, and enjoyable pastime. It can also cause hair loss. I am not sure if there is a more frustrating undertaking than trying to build the perfect load. That being said, and all kidding aside, it does not take a rocket scientist. One of the pitfalls I hope to help you avoid with this article is the effect of hot and cold temperatures on gunpowder.

RCBS Reloading Special 5 Press

Invest in a quality press. It will pay for itself in the long term.

Last winter, the temperatures dropped well below zero for significant time periods across much of the country. This winter has been warmer, but not in all areas. Many hunters still braved the cold armed with handloads developed during the warm summer months. This offered a second lease on life for some critters and a lot of discomfort for others. The pressure difference caused by temperature can cause pressure spikes or drops. Under the right conditions, it can cause an otherwise safe handload to become a dangerously hot load.

The pressure also effects speed of course and while a 130-foot-per-second difference is not going to make a difference at a medium- to large-sized game animal at 100 yards—for those taking on the challenge of 300 to 400 yards, it can play at the edges of the margin. For example, at 200 yards, it could affect your point of impact by a couple of inches. No, this would not throw a perfect shot outside of an 8-inch kill zone, but it reduces it to six or seven inches and increases the odds of a less-lethal wound. For target shooters where millimeters mean the difference in a point, one or two inches is a world.

Typically, the more the mercury rises between loading and firing, the more pressure increase you can expect, but not from all powders. Alliant’s Reloder line is comprised of about 10 different blends, but has a reputation for being susceptible to temperature shifts. On the other hand, Hogdon’s Extreme line is known to be stable across a variety of temperature ranges.

I would not read too much into that last statement though. Powder blends are developed to maximize certain characteristics. Alliant 10 for example is an outstanding choice for my .22-250 varmint rifle. I built it from a Mauser ’96, and matched it with a handload of Alliant 10—song dogs (the one’s I’ve never met) fear it nonetheless. If temperature at range is your main metric, then sure, you would likely favor Hogdon’s Extreme over Alliant Reloder.

revolver with broken cylinder and box of reloaded ammunition

It wasn’t the colt that caused this catastrophic failure; it was putting >357 loads through a .38 special. However, you are just as susceptible to an overpressure failure from temperature.

As an alternative, simply add a line to your shooting data to show the ambient temperature during loading and compare it to when you shoot. A round loaded in cold conditions performs fine in cold conditions. Of course, if you load in your man cave or basement, the temp should be rather stable. However, those who retreat to an unheated barn or outbuilding in winter to do their reloading may want to give extra consideration and record the mercury level.

Testing

Here is a simple way to test the effects of temperature on your handload. First, develop the load under relatively stable temperature conditions. Load 20 rounds to near maximum—with safety in mind—then back it down a bit to give a safety buffer. Place half in the freezer overnight or for a couple of days if you want to be extra sure. If you have a backyard range great, you are ready to go. If not, place the frozen rounds in a cooler and keep them on ice until you are ready to shoot them.

With your chronograph in place, shoot a string of five shots with warm ammunition. Record the variation and inspect the brass to ensure there are no signs of wear, cracked cases or bulging primers. Next, repeat the test with the cold ammunition. Inspect each case after firing to ensure you are not receiving pressure spikes bordering on threatening your safety or damaging you equipment. If you loaded it down from the maximum as earlier suggested, you will be fine—safety first.

Box of rifle cartridges on dash of truck on a hot day

Typically, the more the mercury rises between loading and firing, the more pressure increase you can expect, but not from all powders.

One more note on safety and temperature. Growing up on the left coast, I have shot plenty in extreme heat conditions. For hunting, this isn’t a worry. The cartridges are in the firearm and while hot outside, they are shaded. When firing from an uncovered bench or when the sun is coming in at an angle, the rounds can be more than just hot to the touch.

Conditions such as these can also cause significant pressure spikes. On more than one occasion, I have seen hunters driving country roads looking for bedded deer. In compliance with the law, their rifles were unloaded. However, the hunters chose to place the ammo within easy reach on the dashboard. That was tantamount to ammo subjected to conditions replicating an ant under a magnifying glass.

Accuracy

Temperature is going to affect bullet drop regardless of the power. That is not secret. The colder air is denser, which causes it to offer more bullet resistance and thus more bullet drop. This fact only increases the need to monitor loading temperatures to avoid compounding the problem. However, by spending a few fun hours at the reloading bench, hiding a few cartridges behind the turkey where your spouse will not find them in the freezer and a fun day at the range, you will be safer and hopefully more accurate.

Do you have a cold or hot weather shooting story to share? Tell us about your load work or cold vs. warm weather shooting experience in the comment section.

SLRule

Growing up in Pennsylvania’s game-rich Allegany region, Dave Dolbee was introduced to whitetail hunting at a young age. At age 19 he bought his first bow while serving in the U.S. Navy, and began bowhunting after returning from Operation Desert Shield/Desert Storm. Dave was a sponsored Pro Staff Shooter for several top archery companies during the 1990s and an Olympic hopeful holding up to 16 archery records at one point. During Dave’s writing career, he has written for several smaller publications as well as many major content providers such as Guns & Ammo, Shooting Times, Outdoor Life, Petersen’s Hunting, Rifle Shooter, Petersen’s Bowhunting, Bowhunter, Game & Fish magazines, Handguns, F.O.P Fraternal Order of Police, Archery Business, SHOT Business, OutdoorRoadmap.com, TheGearExpert.com and others. Dave is currently a staff writer for Cheaper Than Dirt!

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Comments (28)

  • Reloading: Load Density

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    […] in some time. Again, I don’t run my handloads wide open. There are differences in performance in cold or hot weather. A load skirting on the edge just may not be completely safe in all environmental […]

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