NEWS | June 10, 2021

Ammunition: How Heat Affects Ammo

Heat doesn’t affect all ammo the same way. Keep reading to find out more.
Mortar Cartridges with Composition B High Explosive Can Exude
When inspecting your mortar cartridges, do you see black or brown crystals, or a yellow or clear oily stain, or a black or dark oily substance oozing from it?  This is a chemical breakdown, called exudation, which indicates that the high explosive fill has degraded, so don’t fire the cartridge! Instead, contact your local quality assurance specialist (ammunition surveillance) (QASAS).

You can find more on this in JMC Safety of Use Message (SOUM) 19-002.
Some Rounds Need Special Ventilation in Hot Weather
Rounds loaded with red phosphorous smoke, mainly the 81mm mortar cartridge M819, can form phosphine gas. This is a very toxic and colorless gas that is either odorless or has a decaying fish or garlic smell. The phosphine gas is also flammable!
Stay safe by ensuring the rounds remain in the original packaging and M819 mortar storage locations have good ventilation. More instructions are in JMC SOUM 14-001:
Firing 120mm Mortars
When the temperature is 120°F or above, it’s more likely a round will fall short of its target if the mortar round has cracked or damaged fins AND if you don’t alternate your propelling charges. Work package (WP) 0024-01 of TM 9-1015-250-10 (Apr 12) and WP-0050-01 of TM 9-1015-256-13&P (Aug 12) tell you to alternate the openings of the horseshoe-shaped propelling charges 180-degrees opposite of the adjacent propelling charge.

You can find more on this in JMC SOUM 11-001:
alternating of the propellant charges
Alternating of the propellant charges
Small Caliber Weapons Warning
It’s normal for small caliber chamber pressure in the ammunition chamber to increase when fired during hot weather. Small arms -10 TMs include warning statements about how hot weather contributes to a cook off (spontaneous combustion of cartridge propellant, usually in a hot barrel), especially if you expose the weapon and ammo to direct sunlight in extreme heat. 

Learn more about how temperatures affect ammo by checking out the following pubs:
  • FM 3-22.9, Rifle Marksmanship
  • FM 3-22.10, Sniper Training and Operations
According to Para 2-81 of FM 3-22.9, high temperatures (and moisture) will affect ammunition and explosives. That’s why you need to take the following precautions:
  • Do not open ammunition boxes until you are ready to use them.
  • Protect ammunition from high temperatures and the direct rays of the sun.
Also, paragraphs 5-94 through 5-101 in FM 3-22.9 address temperature and how it affects ammo. Read these paragraphs for more information:
  • Para 5-99 tells you how changes in daily temperature affects bullet trajectory.
  • Para 5-101 tells you how warmer weather causes bullets to hit higher on the target.
Making Your Shot When It’s Hot

Soldiers, and especially snipers, be aware of the effect temperature plays when making your shot. Para 4-114 in FM 3-22.10 tells you that heat reduces air density. Less air density reduces air resistance and thereby raises the bullet’s point of impact. If you zero at a 500-meter target when it’s 60oF then fire later in the day when it’s 100oF, the bullet will hit about 10 inches higher on that 500-meter target due to the heat of the day and reduction in air density.

Take some time to review Para 2-56 of FM 3-22.10. It tells you when zeroing to take the rifle downrange when checking the target, so the weapon isn’t left laying out in direct sunlight.

Also, be sure to cover the ammo to stop the sun from heating the rounds and changing the ballistics, which happens like this: the heating of the propellant by direct sunlight increases chamber pressure, resulting in the bullet flying faster. The higher velocity causes a flatter trajectory and results in the bullet striking the target slightly higher than a slower moving bullet. Combined with the reduced air density from the day’s heat, even the best marksman will find it difficult to strike the target dead center unless sight adjustments are made to compensate for temperature change.