Effects of Moisture on Rifle Cartridges

Those precisely tuned, high-performance rifle cartridges that were developed in nice dry weather may turn against you when moisture is introduced. Here's how a little water will affect your shooting performance.

Effects of Moisture on Rifle Cartridges

Whether it’s rain or condensation, try to keep the cartridges as dry as possible. Water and high-pressure loads don’t go well together. (Photo by Chris Schaffner)

There’s something satisfying and therapeutic about loading long-­range ammunition that’s consistently accurate and safely at the top of the velocity spectrum. Most often, we conduct our load testing at the range when conditions are near perfect (sunny, moderate temperatures) and shoot off one of the range’s shooting benches. For the most part, it’s a relatively sterile environment.

Those who venture out of this zone and into wet or hot and humid conditions may discover that their perfect match or hunting load exceeds safe pressures when wet. Such was the case when I shot in the Karstetter Memorial Match last April in Wilbur, Washington.

For Problems, Add Water

What I thought were finely tuned 2,899 feet-­per-­second (fps) 139-­grain Lapua Scenar loads for my .260 turned into extremely high-pressure, malfunction-­inducing gun stoppers when wet. Same with my partner’s .260. Just a few days before, we had chronographed both rifles, which were within a few fps from each other, near the top for reasonable velocities but well within safe pressure levels. So we thought.

Wet Ammo and Pressure Issues
Photo by Chris Schaffner

Like typical spring weather in Washington State, it was 38 degrees F, 20- to 30-mph winds and raining sideways. Regardless of attempts to keep our ammo dry, all efforts were fruitless in these conditions. Nothing like lying in the mud, engaging multiple targets under time while cursing your ammunition, which was producing such hard bolt lifts we’d have to come out of position to obtain enough leverage to open the bolt.


On top of that, both large-­rifle primer (LRP) and small-­rifle primer ammunition (SRP) were blowing primers, resulting in bits of metal lodging in the locking lug recesses and case brass ironing itself into the bolt face and flowing into the ejector cutout. This jammed the ejector in the retracted position, resulting in failures to eject.


I spoke with several shooters about this problem. When they know a primer was blown, they would run their bolt to the rear, tilt the firearm up and bang their buttstocks on the ground to dislodge the pieces to prevent problems when chambering another round. As much as this is a pain, it’s far better than running the bolt and trying to gorilla fist it closed with primer debris floating in the action.

Wet Ammo and Pressure Issues
Water is a wildcard in shooting, for no two rounds will perform the same. Instead of flawless operation, water will cause primers to blow, resulting in shards of metal scattered throughout the action. (Photo by Chris Schaffner)

Aside from rain or snow, also keep condensation in mind, which forms when moving your ammunition from an air-­conditioned environment to hot temperatures. You’ve seen this condensation on your scope and other optics. Once outside, flip the lids open on your ammo boxes to allow condensation to evaporate or wipe each round off after it’s been exposed to the ambient temperature for a period of time. Additionally, ensure your chamber is free of oil or solvents.

Why It Happens

When a wet cartridge is introduced into a chamber, the round brings water molecules with it. Since water is virtually incompressible, this limits the internal chamber space and the ability for the brass to expand — hence increasing chamber pressure.

If the ammunition is near the top of the pressure scale to begin with, unsafe pressure levels are almost guaranteed. If the load is mild or moderate, then pressures will increase but perhaps not to dangerous levels.


When loading ammo, consider other factors that could exacerbate a wet ammunition occurrence. Examples include loading near maximum pressure/velocity with temperature sensitive powders, testing in cool temperatures and shooting in hot temperature and high humidity conditions. I’ve seen these conditions skyrocket pressures with the same dangerous results.

I’m not the “hold my beer, watch this” guy of yesteryear, but there are certain things I need to find out for myself. One of which was how would my moderate-­velocity 6.5 Creedmoor 140-­grain Barnes’ Match Burner loads perform when wet. (Disclaimer: do not try this for any reason!)

Under Pressure

With a box of loaded ammo, I chronographed 10 dry rounds and then did the same with 10 more that were wet from pouring water on them while in the magazine. As expected, the wet rounds produced noticeably higher pressures, including stiff bolt lifts, pronounced ejector marks but not blown primers.


Wet Ammo and Pressure Issues

One would think that along with increased pressures would come significant increases in velocity. Actually, just the opposite occurs. Water on the case no longer allows the brass to expand evenly to fill to the chamber walls. In the normal ignition pressure sequence, energy expended in accelerating the projectile down the barrel is now channeled towards the rear of the case into the bolt face.

The chronograph results speak for themselves: higher pressure, lower average velocities and extreme spreads and standard deviations three-­times that of dry ammunition. One could surmise that in addition to increased pressures, wet ammo will also cause inconsistencies in velocities. No two rounds will have the same amount of water on them or in the same location, so they will react differently under ignition. Hopefully you won’t experience these issues, but if you do, perhaps this will give you an insight of the cause and remedy.

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