Ammunition & Velocity – Hot vs. Cold Car

Its commonly understood that ammunition that is hot will propel a bullet to a higher velocity than those at colder temperature. In fact, the 1:7 twist rate for the M16 was chosen to be able to stabilize the tracer round at sub-arctic temperatures.

The Wound Channel did a short experiment using ammunition left in the hot sun versus those kept in an air conditioning vent. The difference of only 100 degrees is nearly 100 FPS, which with a 5.56 can be a major difference in lethality, especially at longer ranges. Its good news for our boys in the Middle-East who are plagued by high temps and something for them to look out for when up in the Afghan mountains.

Nathan S

One of TFB’s resident Jarheads, Nathan now works within the firearms industry. A consecutive Marine rifle and pistol expert, he enjoys local 3-gun, NFA, gunsmithing, MSR’s, & high-speed gear. Nathan has traveled to over 30 countries working with US DoD & foreign MoDs.

The above post is my opinion and does not reflect the views of any company or organization.


  • BattleshipGrey

    Interesting. Just don’t get too used to it in case you have to fight in the arctic circle, which is actually looking like more of a possibility.

    • Jack

      It’s ok. By the time we’re fighting over it, it won’t be “arctic” temps like they’re supposed to be.

      • USMC03Vet

        Ok Al Gore

  • Sianmink

    “Its commonly understood that ammunition that is hot will propel a bullet to a higher velocity than those at colder temperature.”
    I’ve found this is more true for rifles than for pistols. I’m not sure of the properties behind it, but I’ve found the exact opposite to be the case with some pistol loads. They’ll get soft at high temperatures, and blazing at below freezing.

    • marathag

      During WWII, Soviets reported that the LendLease 45 Auto couldn’t reliably penetrate the winter clothes of the Finns in the Winter months.

      • iksnilol

        But… But stopping power?

        • marathag

          Other post modereated, due to link. Google on

          Dmitriy Loza and Sherman or iremember(dot)ru/en/memoirs/tankers/dmitriy-loza/

          – Dmitriy Fedorovich, there were personal weapons in each Sherman that arrived in the USSR, Thompson submachine guns (also known as the Tommy gun). I read that rear area personnel stole these weapons and that few tanks arrived in units still equipped with them.
          What kind of weapons did you have, American or Soviet?-

          Each Sherman came with two Thompson submachine guns, in caliber 11.43mm(.45 cal), a healthy cartridge indeed! But the submachine gun was worthless. We had several bad experiences with it. A few of our men who got into an argument were wearing padded jackets. It turned out that they fired at each other and the bullet buried itself in the padded jacket. So much for the worthless submachine gun

          He was a Hero of the Soviet Union, equipped with LL M4 Shermans.

          The M4 he liked.
          Thompson, not so much

          • iksnilol

            Was meant as a joke since I am not much of a fan of .45 ACP. Interesting story, is it possible the cold resulted in less velocity? .45 ACP has pretty low velocity already.

            I wonder what their bad experiences were? Malfunctions?

          • marathag

            I recall some talk of that being a problem in Korean Winters too.

            I know some shotgun powders can do a lot lower pressure in cold, so is plausible to me that it could happen with pistol powder

      • Ben Loong

        Wait, what?

        That statement makes no sense. The Winter War ended a year before the Lend-Lease policy was even enacted. Why would the Soviets have used .45 ACP guns when they fought the Finns?

        • marathag

          They fought the Finns right thru 1945, when they were ‘co-belligerents’ with Nazi Germany. over 140,000 Thompsons were sent over

          • Ben Loong

            Huh, I was previously not aware of the Continuation War.

            I always assumed the Soviets had bigger problems than the Finns by the time the Germans were rolling over them.

  • Concept

    Less than a 3% difference which is nothing when you’re talking about 3,000+fps average. And you’re talking two extremes as well. One was at probably 120+ degrees and the other at 50 degrees and the difference is what amounts to a rounding error.

  • Lance

    In the Afghan maountains you do get a BIG swing in temps over 100 F in the summer -20+ in the winter.

  • William

    Thanks so much!!!

  • W.P Zeller

    Actually, certain pistol powders go slower when hot and faster when cold.
    Specifically, Winchester Super Target is notorious. Those near-max loads made back in June can be very stressful if fired out on the range in January.