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June 7th, 2023

How Ammo Temperature Affects Velocity, Pressure, and POI

Sierra Bullets Ammunition Ammo temperature temp test hot F-Class Ammo cold
In this .308 Win test, 70° F ammo shot 96 FPS slower than ammo heated to 130.5° F. And the 130.5° ammunition was 145 fps faster than ammo taken right out of the freezer (at 25.5° F). That’s a huge difference…

summer heat ammunition temperature velocitySummer Solstice is just two weeks away, so most parts of the country will soon be encountering peak summer heat. Some ranges in the Western states have already recorded temperatures well over 100 degrees F during matches. When dealing with extreme summer heat, you should make a serious effort to keep your ammo at reasonable temperatures. When possible, keep ammo in a cooler in the shade.

Never leave boxes of ammo out in the hot sun. Even with powders advertised as “temp stable” you can see significant velocity increases when ambient temps reach 90 degrees and above. This article explains how temperature extremes (both hot and cold) can alter bullet velocities. The velocity differences between very cold ammo and very hot ammo can be very large, as this article explains.

EDITOR’s NOTE: The Sierra tester does not reveal the brand of powder tested here. Some powders are much more temp sensitive than others. Accordingly, you cannot extrapolate test results from one propellant to another. Nonetheless, it is interesting to see the actual recorded velocity shift with ammo temperature variations in a .308 Win.

Written by Sierra Chief Ballistician Tommy Todd
This story originally appeared in the Sierra Bullets Blog
A few weeks ago I was attending the Missouri State F-Class Match. This was a two-day event during the summer and temperatures were hot one day and hotter the next. I shot next to a gentleman who was relatively new to the sport. He was shooting a basically factory rifle and was enjoying himself with the exception that his scores were not as good as he hoped they would be and he was experiencing pressure issues with his ammunition. I noticed that he was having to force the bolt open on a couple of rounds. During a break, I visited with him and offered a couple of suggestions which helped his situation somewhat and he was able to finish the match without major issues.

He was shooting factory ammunition, which is normally loaded to upper levels of allowable pressures. While this ammunition showed no problems during “normal” testing, it was definitely showing issues during a 20-round string of fire in the temperatures we were competing in. My first suggestion was that he keep his ammunition out of the direct sun and shade it as much as possible. My second suggestion was to not close the bolt on a cartridge until he was ready to fire. He had his ammo in the direct sunlight and was chambering a round while waiting on the target to be pulled and scored which can take from a few seconds to almost a minute sometimes.

This time frame allowed the bullet and powder to absorb chamber [heat] and build pressure/velocity above normal conditions. Making my recommended changes lowered the pressures enough for the rifle and cartridge to function normally.

Testing Effects of Ammunition Temperature on Velocity and POI
After thinking about this situation, I decided to perform a test in the Sierra Bullets underground range to see what temperature changes will do to a rifle/cartridge combination. I acquired thirty consecutive .30 caliber 175 grain MatchKing bullets #2275 right off one of our bullet assembly presses and loaded them into .308 Winchester ammunition. I utilized an unnamed powder manufacturer’s product that is appropriate for the .308 Winchester cartridge. This load is not at the maximum for this cartridge, but it gives consistent velocities and accuracy for testing.

I took ten of the cartridges and placed them in a freezer to condition.

Sierra Bullets Ammunition Ammo temperature temp test hot F-Class Ammo cold

Sierra Bullets Ammunition Ammo temperature temp test hot F-Class Ammo cold

I set ten of them on my loading bench, and since it was cool and cloudy the day I performed this test I utilized a floodlight and stand to simulate ammunition being heated in the sun.

Sierra Bullets Ammunition Ammo temperature temp test hot F-Class Ammo cold

I kept track of the temperatures of the three ammunition samples with a non-contact laser thermometer.

The rifle was fired at room temperature (70 degrees) with all three sets of ammunition. I fired this test at 200 yards out of a return-to-battery machine rest. The aiming point was a leveled line drawn on a sheet of paper. I fired one group with the scope aimed at the line and then moved the aiming point across the paper from left to right for the subsequent groups.

NOTE that the velocity increased as the temperature of the ammunition did.

The ammunition from the freezer shot at 2451 fps.

Frozen FPS

The room temperature ammunition shot at 2500 fps.

Room Temperature FPS

The heated ammunition shot at 2596 fps.

Hot 130 degree ammo

Sierra Bullets Ammunition Ammo temperature temp test hot cold

The tune window of the particular rifle is fairly wide as is shown by the accuracy of the three pressure/velocity levels and good accuracy was achieved across the board. However, notice the point of impact shift with the third group? There is enough shift at 200 yards to cause a miss if you were shooting a target or animal at longer ranges. While the pressure and velocities changed this load was far enough from maximum that perceived over pressure issues such as flattened primer, ejector marks on the case head, or sticky extraction did not appear. If you load to maximum and then subject your ammunition to this test your results will probably be magnified in comparison.

Sierra Bullets Ammunition Ammo temperature temp test hot cold

This test showed that pressures, velocities, and point-of-impact can be affected by temperatures of your ammunition at the time of firing. It’s really not a bad idea to test in the conditions that you plan on utilizing the ammo/firearm in if at all possible. It wouldn’t be a bad idea to also test to see what condition changes do to your particular gun and ammunition combination so that you can make allowances as needed. Any personal testing along these lines should be done with caution as some powder and cartridge combination could become unsafe with relatively small changes in conditions.

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June 7th, 2023

The Rifle That Shot 3000 Bullseyes — Sam Moore’s Win Model 52

Samuel Sam Moore Calvin Coolidge 3000 Bullseye NRA Museums

Here’s a rifle that earned a Presidential medal and a visit to the White House. This rifle has a unique heritage, having been used to shoot 3000 consecutive bulleyes in a single day. The year was 1926 and a high school shooter named Sam Moore hoped to set a record. With his trusty Winchester Model 52 rifle in hand, Moore fired 3,000 rounds downrange, only stopping when his rifle became too hot to hold and daylight was fading fast. But he had fired 3,000 consecutive bullseyes in NRA Junior Rifle competition (target at 50 feet). The event, which set a world record, received national attention.

Samuel Sam Moore Calvin Coolidge 3000 Bullseye NRA MuseumsMoore was summoned to Washington, DC on April 26, 1926 to meet President Calvin Coolidge. At the White House, President Coolidge met the young marksman and presented Moore with a gold medal. The engraving on the back reads: “Presented to L.S. Moore by the President of the United States [on] behalf of the National Rifle Association. Junior Rifle Corps World Record — 3000 — consecutive bullseyes.”

Editor: Consider how times have changed. In 1931 the U.S. President was proud to honor a junior marksman for a great achievement. Today the White House condemns everything gun related. And the State of California has passed legislation imposing $25,000.00 fines for promoting youth marksmanship (AB 2571).

Marksman Moore Went to the U.S. Naval Academy and Served in WW II
After high school, Sam Moore earned admission to the U.S. Naval Academy, and graduated in 1931. Later, Moore helped develop the Fairbairn-Sykes fighting knife, served in WW II as a USMC aviator, and maintained his interest in shooting until his passing in 1982. Moore’s rifle and engraved gold medal were donated to the National Firearms Museum by his son David.

Photos and story from NRA Museums Facebook Page

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