How Ammo Temperature Will Affect Shooting Accuracy

The last time we visited the Hackaday shooting range we were all psyched up to get the right posture, breathe correctly, lower our heart rates and squeeze the trigger at exactly the right moment that the wandering cross hairs align with the target ……. and lastly accommodate the inevitable recoil. But never did we think to check the temperature of our ammo! Ok, temperatures aren’t likely to vary that much there unless the range cat chooses to lay down on top of the ammo box, but out in the wilderness the temperatures can easily vary by up to 30 degrees, which would certainly be a problem.

If we take a quick look at what’s happening on Johnny’s Reloading Bench  we get an in depth comparison of different powders at different temperatures, with data being collected via a bullet velocity radar. If nothing else, it’s interesting just to get a peep into the mysterious world of ‘Reloading’ where every one of the tiny kernels or ‘balls’ of powder make a difference and different powders require particular primers to make them burn properly.

Just to make it clear, bullet speed makes a big difference to the trajectory, especially at long distances. For example, if the bullet were to travel at close to the speed of light, there would be almost no trajectory at all and the shooter would not have to adjust the vertical aim for distance. Normally, we have to aim upwards to hit the target:

It may be that we ‘zero in’ our sights at room temperature, but then end up actually shooting the firearm on a cold, frosty morning with cold ammo, and given what we have now learnt from the video, we could now make a small adjustment for that eventuality, depending on the particular ammo we are using. Johnny’s video is after the break:

Strangely, we’ve only ever talked about Reloading twice before on Hackaday, auto-trickler-gently-doles-out-powder-to-assist-reloading/case-feeder-makes-your-ammo-reloading-more-efficient/

32 thoughts on “How Ammo Temperature Will Affect Shooting Accuracy

    1. So that the FBI and probably the bulk of the US Federal law enforcement can practice before blowing things up out here. So that folks who shoot competitive can stay that way when the weather outside resembles garbage.

      1. Also because they can be put in much smaller and more crowded areas, because they are quieter (outside), because they manage toxic contamination better, because they are not subject to range intrusion hazards, etc, etc.

    1. And how exactly would it have a greater impact then the powder temperature? Unless we’re talking uneven heating where it obviously bends the barrel in some way, the size change is microscopic for the diameter and meaningless for the length.
      Whereas powder temperature will vary muzzle velocity by several % if we allow for differences of 20+ degrees.

      1. Because the heat is transmitted to the powder by the brass really well and quickly. In fact, firing faster than recommended for a barrel can get it so hot that new rounds “cook off” as the bolt is closed. In movies and TV – and in emergencies – automatics like M16 or AK are fired like crazy. The rifle carried by US infantry, the M4, is rated at 750 to 950 rounds per minute, but it only takes 140 rounds of rapid fire to reach the cook-off point. So there are limits like a few seconds of rapid fire followed by a burst (it has 3-shot burst mode) ever few seconds and at the 20 second mark drop to a several seconds between shots or bursts then maybe a burst every 10 or 15 seconds – I’m guessing here.

        The takeaway is that less than 15 seconds of rapid fire and the ammo goes off as it enters the chamber. For some reason Hollywood guns never jam on full auto.

          1. Very impressive. And expensive in gun and ammo. I got the numbers from the M4 manual or something like that. Maybe steel cases are better for this. Anyway, he doesn’t take long to start cooking but definitely longer than I expected.

  1. I am inclined to think it has more to do with the thermal properties (expansion / contraction) of the bullet and casing than the powder itself. The powder would need to to be tested itself (somehow) to determine causality.

  2. I’d like to know his “warmed up” barrel temp at the beginning compared to the end of his test. . Also if he continued rotating the loads in the same order. Preferable test would be ambient temp, pressure, and humidity data with cold barrel temps at each shot. But I get it, without 18 hours in a climate-controlled indoor range for 25 rounds…. Interesting results.

  3. I was going to say, the receiver and barrel go through a pretty quick warm up of more than a few degrees. I think the temp of the ammo is a factor, but I think it is overshadowed by other things that change. When you site in a gun I have never seen anybody let it cool down first. By the time you are “there” usually the barrel is pretty hot in fact, but when you go to shoot the first shot, the barrel is cold. If you are hunting targets that is not an issue, if you are hunting game, the first should be the last shot. When I used to hunt big game I would take my muzzle loader in fact. You get one shot at the prize.

  4. The temperature of the ammo is effectively a moot point in shooting. The temperature that matters is the barrel temperature. You can store your ammo in a cooler filled with dry ice but once you chamber the round, it will rapidly attain the temperature of the barrel… unless you are shooting a revolver. Your first shot will, effectively, be at room temperature. The temperature of the second shot, and all successive shots, will depend on the ammo being used, rate of fire and the rate at which the barrel can dissipate the heat.

    1. It seems to me [a non-shooter] that the barrel temp has a twofold effect – the ID of the barrel would reduce, increasing friction of the round as it travels thru the barrel, and reducing the velocity of the round, and second, heating the air in the barrel, and thereby reducing the density of the air the round has to displace as it travels thru the barrel, thereby increasing the effect of the powder accelerating the round down the barrel.

      1. This is true, although the decreased density of air is negligible compared to the increased friction of the rifling against the bullet.

        While that affects velocity, the increased temperature will cause a slight droop in the barrel, especially on longer and heavier barrels, changing the initial trajectory. Tapers, flutes, and barrel mass all go into mitigating those effects.

        Length will also change, which can have very different affects on which direction the muzzle deviates depending on barrel length, wall thickness and taper, and locations and types of attachment points.

        If you want to think about non-thermal factors, just take a moment to ponder resonant frequencies :)

    2. Cold bore versus warm barrel tests have shown no measurable difference in precision shooting, though there are plenty true believers who will dispute that. The popularity of Precision Rifle Shooting (PRS) and new affordable chassis rifles from the major manufacturers has caused a lot of focus on this and some serious investigation. Some powders show a temperature effect but most modern ones are negligible. After all, they are all basically nitrocellulose soaked in nitroglycerine with variations in grain size and shape and coatings that control burn rate. The powder gets hot really fast. The difference in energy hot versus cold at time of ignition is minute. The propellant itself is a whitish translucent stuff. The coatings give it the gray or black color.

  5. Targetting / plotting computers of the analogue variety have been taking into account ambient and propellant temperature since the 1930’s onboard battleships and even field artillery.
    I guess shooters are expected to DIY :)
    Part of the fun/skill.

    1. And the (official) core use for the very first digital computers was PDE solution approximations for the calculation of artillery tables, including those factors. Unofficial of course was crypto, and un-unofficial was general nerdiness, hacking and academic shenanigans, which is both why and how this page even exists.

    1. This. To get a 10-gram bullet moving at mere 0.35c, your powder charge has to be roughly equivalent to the Little Boy bomb. That just might cause all sorts of … interesting effects.

  6. Peltier junctions are the Arduinos of thermal management: Overused for comically-complex solutions to simple problems. :)

    The problem of thermal stability in gun barrels was solved with several simple solutions 100 years ago. One, wrap a water jacket around it (water jackets were the Arduino of the early 20th century), a la Maxim and hundreds of derivatives and variants. Two, make the barrel thick enough that thermal effects are negligible, aka “heavy barrel”. A modern example is the Remington 700P and military derivatives. Three, make the barrel quickly replaceable in the field, a la the MG 42. Four, for any and all situations, just don’t shoot fast enough for heat to build up!

  7. The temperature sensitivity is not a barrel or powder problem but mainly a primer problem.
    When I shoot 300m free rifle matches outdoors at -10C I need magnum primers or I have 5% of the shots with delayed ignition and low on target. Match season is from April till September here so this happens rarely.
    Many shots have bad ignition without you noticing it but the average spread is much higher. For hunting, cartridges with small rifle primers and a relatively large case such as the 6,5×47 are not recommended because of this. (I don’t hunt so I don’t care much.)

    1. Delayed ignition of nitro because of primers? Have you tried bench rest primers? And -10C is warm compared to winter hunting in the Yukon or the American West. Maybe change powders to one that gives a compressed load.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.