Competitive Shooting

Temperature, Cartridge Pressure, Accuracy — The Effects of Temperature

revolver with broken cylinder and box of reloaded ammunition

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.

[dave]

The Mission of Cheaper Than Dirt!'s blog, The Shooter's Log, is to provide information—not opinions—to our customers and the shooting community. We want you, our readers, to be able to make informed decisions. The information provided here does not represent the views of Cheaper Than Dirt!

Comments (28)

  1. I am not sure what Dave calls a pressure increase due to temperature differences between loading and shooting. Is he referring to the pressures within the barrel when the round is fired or pressure due to the expansion (or contraction) of the powder within the cartridge due to temp changes?
    I’d be inclined for the first option, but what causes this difference? There are no loading tables that I know of that include temperature as a loading parameter. While I believe there is a difference, IMHO I doubt that it presents a safety hazard, otherwise manufacturers would have already published the data.

    1. Pressure in the barrel from the round going off… & believe me it is dangerous using temperature sensitive powder & not knowing it!.. If u look at mostly alliant powder it’s all temperature sensitive!!! Meaning the fluctuations in ambient temperature has a major effect on the velocity of the round!!!… Look it up there’s a graph showing the difference… I’ve learned my own lesson with reloader 22 in my 25-06 pump… Had to replace my ejector pin & spring twice… I stick with hodgdon extreme powders now!!!… Be safe! & Happy reloading!…

    2. @ Javier Guzman.

      “Dave’s” probably referring to the process of Actually Mating the Bullet to the Propellant Cartridge. Before you Mate the Two, the Air Pressure inside the Cartridge is at “1” Atmosphere. As you Mate or Crimp the Projectile with the Propellant Cartridge the Pressure Actually Increases Exponentially. As you CRIMP the Bullet Inside the Cartridge, You’re INCREASING the Air Pressure. The Deeper you Seat the Bullet, the Greater the Pressure…

  2. I am having a hard time understanding this article. Is it stating that rounds LOADED at different temps will have different POIs, OR is it stating that rounds TESTED at different temps will have different POIs? Is it stating that if I load a round when my garage is 10 degrees, that it will have a different POI than a round loaded when it is 80 degrees, even if they are both fired under the same conditions/temp? OR is it stating that rounds from the same batch will shoot differently based on temp out?

    Either I am reading this incorrectly (completely feasible), or this article needs a little more clarity.

    1. The temperature of the cartridge / propellant at the time of firing is the determining factor. With some propellants the ‘burn-rate’ varies with temperature. Then there are also propellants that are formulated to minimize the burn-rate variation with temperature. The only way to check how your handloads perform under both cold and hot conditions is to actually range test them out to what you consider maximum range for hunting – under conditions similar to those you might be likely to hunt in. Have fun….. The more you learn (and practice), the better you’ll get.

    2. Poor wording on his part. He’s from Pa. so cut him some ignorance slack. If you load bullets in winter; say 5.7 as a great example… while using a chrony… you V may be 1600 fps on a 20 degree day while working up loads. Yet on a day with 80 degree in summer… you may end up with 2600 fps… thus breaking gun parts or worse. The moral of the story is always check your workup on 60 degree days (f)… so you’re not surprised (chrony). Or use temp stable powders; which are very few. If you want temp and flame… good luck. You can buy temp and add flame yourself by adding 5 percent by volume wet chem and then dry; but you will find no one… I mean no one who will help you out in this venture. Much like mixing powders. When you get to advanced stuff; you need to not only measure V out of your barrel; but have a pressure trace on the breech to measure pressure spikes on the metal itself. It’s a 250 dollar measuring device; but most are not adept at such things and will usually blow their thumb off first. Finally, it’s bad enough with law enforcement and out socialist modality; so when you become advanced… expect much resistance. Even if your the pinnacle of society; they’ll be stacking your record with red flags for that grainy day.

  3. This is an Interesting note. A bit of experimentation is always in order.
    Yes air density is important. You might also note the following(one of many): http://chemwiki.ucdavis.edu/Core/Physical_Chemistry/Kinetics/Modeling_Reaction_Kinetics/Temperature_Dependence
    Most chemical reactions do show increasing rate at higher temperature. A doubling of rate every 18 deg. F is a one thumb rule for biochemical systems. Duly noting the effect of organic solvents, the startling thing for me is the extent to which manufacturers have stabilized both powers and ignition against external factors.

  4. Interestng article.
    So how exactly does temp change effect powder to cause pressure spikes? Expansion/ contraction? If so, shouldn’t there be a coefficient (for determining volume)”for temp variations in reloading? Contraction/expansion won’t change composition or its nature and I question if volume change due to expansion would be significant since it should not change the composition or nature of the powder.
    Best practice is to load to best documented loads for your purpose and mind the defined margins of your powder of choice. Be safe and enjoy the shooting sports.
    I have heard that extreme low temp will negatively effect a cartridges performance and so would think that high temp might effect powder if the temp is at or near its flash point, but a steel tool in the desert sun only reaches about 160F.
    Like I said, I find the article interesting.

    Sometimes its the propellant that is corrosive and not the lubricant and sometimes, some CLP’s contain a clorinated solvent that is corrosive when in contact with water.

    1. @ Bpenn.

      By “Thermal Dynamics”, Constant Expansion and Contraction of Propellant Cartridge Casing Produces HEAT. Over Time it can Degrade the Propellant inside the Cartridge. STEEL is the WORST, because of the Thin Wall Construction of the Cartridge Casing. BRASS is Less Dynamic as far as Expansion and Contractions, and FAR Safer. And is Static Electricity Neutral, UNLIKE Steel Cartridges. NiCup or Cupronickel ( Nickel-Copper alloy) also a Good Substitution Choice…

  5. It seems as though the writer has confused temp at loading with temp at shooting. This matters! The difference of temp at origional testing vs the temp at shooting (ie hunting) is what matters, not temp at loading vs temp at shooting. Try again.

    1. @ Clinton.

      I don’t think that was the Problem? I think “DAVE” Oversimplified the Reloading Procedure. He was Probably trying to Explain it on a Novice Level of Experience. And it Got Lost In the Translation Process. What makes sense to one person, may have a Different meaning to another. It took me more than 2-hours to write my piece of the process…

  6. Temperature and Humidity is your ENEMY, when Storing Ammunition. Sore what you Absolutely Need, and NOT FOR WAR. Old Ammo, is 30-years or More. Propellants, start to Break-Down after about 10-years. That INCLUDES “Hg(ONC)2” or Mercury Fulminate. The Higher the Temperature, the Faster the Break-Down Time. And what ever you do, KEEP “WD-40” as far away from Ammunition Propellants as you can. It’s a Corrosive Solvent, “Mercury Fulminate” is also Highly Corrosive. And the Last Thing You Want To Do, IS MIX THE TWO. Treat the Two like, you would a SNAP-10A Nuclear Reactor. BURY AS DEEP AS YOU CAN…

    1. @Secundius

      Great comment and very helpful. I think the very oldest ammo I still have around dates back to around 2003. I go through pistol ammo pretty quickly, but not as much with rifle ammo. It’s stored in ammo cans with desiccant packets, but some of it isn’t in a climate controlled area. Time to start rotating it a bit more.

      I was unaware of the nature of WD-40 and ammo.

    2. @ Mikial.

      Back in the Mid ’80’s, I had a Open Can of WD-40. Which I accidentally spilled on my glasses., They were Lexan Framed with Polycarbonate Lenses. I went to get some Rags to clean the Mess, on the Counter Top and had a “Slimy Black Spot” where my Glasses were and it Eat a 3-inch Hole in you Floor.

      A Friend of mine had a Near Fatal Experience with WD-40 MIXING into the Propellant Compound that he was mixing. Not an Explosive One, but by Asphyxiation One. He also had to replace the Dry Wall, because the Noxious Impregnated the Walls. But SOME Propellants WILL Explode or Burst Into Flames, when coming into contact with WD-40…

    3. Wow, thanks, I knew it was a lubricant but I had no idea it was that nasty. I actually have a can sitting in my room where i clean my guns, although i never use it on any of the guns. It was mainly there for squeaky door hinges and the like. It won’t be there in a few minutes.

      Speaking of things dissolving, I needed a small brush while cleaning a rifle one day, and grabbed a cheap plastic handle toothbrush to use. After I was finished I left the brush in the top of my cleaning kit and it there fro a few days. When I came back all that was left were the bristles. The entire handle and head had dissolved completely.

    4. I just found and shot a box of Ted Williams, Sears and Roebuck 30-30 in my late fathers drawer. Probably vintage 1970’s. The key to maintaing ammo is temp, lubricant and moisture free.
      Don’t throw out aged ammo, shoot it. And definitely under any circumstance don’t bury it. When in doubt take it to a range or gun shop, they’ll be happy to shoot it for you.

    5. One thing to note; WD-40 is not a corrosive solvent. The one reason it’s bad to clean or store ammunition in WD, is because it has such a low viscosity, and a very high penetration rate. As such, it’s very good at working into even the smallest of crevices. It can thus work its way into the round, fowling the powder and primer, and rendering the round inoperable.
      If it were a corrosive solvent, it would attack metal, including brass, much as brake cleaner does with aluminum (such as the white oxidized powder that forms when you clean a carburetor for instance), or as it does when it hits steel, and flash rust is formed almost instantly. WD specifically states that it’s safe for use on all metals, and anyone who has ever used it to prevent rust on their tools can attest to this.

      Now, using true corrosive solvents, such as brake or shotgun cleaner, to clean old rounds IS a BAD idea. First of all, if the brass is already corroded, it has more than likely been weakened to some varying degree, and has the possibility of failing by the way of a ruptured case.

      As well, the corrosive solvent compounds this problem, as it will attack the brass, further weakening it. As well, as you stated about mercury fulminate, I wouldn’t want two corrosive solvents near each other either.

    6. Well, I do have to modify one thing I said. Out of curiosity, I just re-read the MSDS for WD-40 for the first time in a long time, and even though it isn’t a corrosive solvent, it CAN react with Strong Oxidizers causing a thermal reaction (specifically heat, as outlined by the Material Safety Data Sheet). Now, as we know, in order for something such as gun powder and smokeless powder to burn the way it does, it needs an oxidizer. In Black powder, this could be saltpeter, or in modern smokeless powders, any of a number of chemically bound oxidizing agents. So, there is a strong possibility WD could cause a reaction because of this fact after it initially penetrates the round, but not because it’s a corrosive solvent. It really depends upon whether the oxidizing agents in modern smokeless powders can be classified as strong oxidizers in this case.

      Also, WD-40 eats plastic (much as some bug sprays will), because of the petroleum distillates in it. Ask my spotted up glasses how I know?

    7. Not sure what kind of propellants you are using…. Mercury fulminate was used as a priming compound but not for propellant. BTW, the United States Army discontinued use of mercuric priming mixtures in 1898. So you must be talking some really old or foreign ammo. Modern propellants and primers, when stored properly, DO NOT break down and will last an indefinite period of time. Alliant (formerly DuPont) has a stash of Unique powder that is over 100 years old. Periodically, they take a bit out and test it. To this date, they have not noted any decomposition of the propellant. Personally, I have a 8lb can of IMR4895 that is at least 50 years old. I load a few cartridges a year from it and it works just as well as the day I purchased it. WD40??? Good grief! Why on earth would you have that around your reloading bench??? It serves no purpose. Not good to lubricate cases or machinery.

    8. @ Diverjay.

      Because is ALSO Lubricates. MOST people Think of WD-40 as a Lubricant and a Solvent, AND SOME seem too think that a Lubricant and Solvent are the SAME THING. WD-40 was created in 1953, by Rocket Scientist for Rocket Scientist. So in this Case, it IS all about Rocket Science…

    9. @ Divejay.

      NOT THE Propellant Sir! The Percussion Cap at the Base of the Propellant Cartridge, That Set’s of the Propellant Charge. THAT “Mercury Fulminate”…

  7. Awesome article!!! I started reloading, a friend got me started with a great accurate load in my 25-06 in the winter… I then in then duplicated it, went to work in summer & my gun & ammo Sat in the baking vehicle for 8 hours I go groundhog hunting, shot 1 & it smoked like a muzzleloader, kicked like a 300win mag & the brass stayed stuck in my bolt face!! Jammed my ejector pin back in the breech bolt!!! Taught me about pressure sensitive powder really quick!!! NO MORE RELOADER 22 FOR ME!!!

  8. As a technical article – excellent, and with”weather extremes” now being touted as part of “climate change” – very timely.
    I do however have a small problem. I am a sailor – each time I came to the word ‘mercury’ my mental image was a barometer – atmospheric pressure, not a thermometer – temperature. My hangup, not yours. Your lesson still came through loud and clear.

  9. This was a fascinating article and I enjoyed it very much. I’m not a reloader, but I understand the significance of fine tuning reloading specs, and I think it’s very cool. I do have some of my .357 and .44 Mag ammo for my Desert Eagles custom loaded by some people I trust because Desert Eagles are a very heavy action gun and need good loads that are hotter than loads for revolvers or some of the lighter .357s out there today.

    But for me, as a defensive shooter who has served in combat zones (NOT as a sniper) and carries everyday, I have to wonder if any of this really makes that big a difference in regards to my own situation. I understand the need for consistent results over long ranges with solid bench rests, but I don’t think all this will mean a lot in my load-out for EDC.

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