Rain, humdidity & increased pressures...

[clyancey]

I've seen it said that guys start blowing primers etc when shooting matches in the rain.

Is it simply water working its way into the chamber/bore reducing chamber capacity and increasing bullets resistence through the bore? Or what exactly is going on here? Can high humidity effect pressures as well?

Thanks.

 

[bschneiderheinze]

I think I had this problem last weekend. everything was fine in the morning then it starting raining and my rifle got wet. First stage I blew out three primers.

 

[_Windrider_]

Excerpt from here https://www.primalrights.com/library/articles/understanding-pressure


“Water
Wet ammunition is a very common cause of over-pressure. Rain storms, water splashed while running, condensation, and many other things can cause wet ammunition. This is a problem for two main reasons: The first is that water is not very compressible. Contrary to popular belief, it can be compressed, but it has a very high resistance to compression. What this means is that when water is put under extreme pressure the volume of space it occupies will not be reduced significantly. This is important to us because if water gets on our rounds, and those rounds go into the chamber wet, then water will be occupying space in the chamber. When the cartridge is fired, that water can not be easily compressed, and effectively reduces the size of our chamber directly proportionately to the water volume. That reduced size results in less space for the pressure created by burning powder to occupy, thereby increasing pressure. The second reason water being on our ammo is a problem is friction, or described more appropriately, a lack thereof. Water is slippery. One of the core functions of the brass is to expand and grab hold of the chamber walls, spreading the pressure across the entire area of the brass against the chamber. If the brass is not allowed to grab hold of the chamber walls, then all of that pressure is transferred rearward onto the base of the case, the bolt face, and subsequently the bolt lugs and action. If water is in between the brass and the chamber walls, even very light and mildly loaded ammunition will cause severe pressure signs. The ability of the brass to grab onto the chamber walls is paramount to the proper functionality of the firearm. “

Humidity could be an issue if you are at the upper limits of pressure in your load development but it’s more the presence of actual water that’s at issue.

 

[Lowlight]

Water does not compress, it creates high pressure in the chambers

 

[clyancey]

I hadn't even considered the brass to chamber friction being reduced. Makes sense. Thanks TacticalDillhole and Lowlight.

 

[CShooter92]

Reviving this thread from the dead with a simple question.

Will water in the chamber show extreme pressures on a load regardless of how hot the load actually is? Say for example a load 2 grains off book max vs. a load at book max?

 

[AIAW]

Reviving this thread from the dead with a simple question.

Will water in the chamber show extreme pressures on a load regardless of how hot the load actually is? Say for example a load 2 grains off book max vs. a load at book max?

Usually, yes. One aspect is the lost chamber volume from the water (which could be negligible overall), but the lubrication the wet brass/chamber walls has will cause bolt-thrust. You can get pressure signs rather easily with this - same situation if lube isn't cleaned off well, etc.

 

[msstate56]

I’ve seen the pressure spike in the rain with factory ammo. I was in a match that required a bolt back, empty chamber, round in hand 300 yard cold bore shot. In the pouring rain. My round got wet, and when it went off, I new something felt different. Round impacted about 3” high. Very stiff bolt opening. When I extracted the case, saw a flattened primer and a nice “water spot” on the case wall. Ever since then I try really hard to keep my rounds dry.

 

[LC 6.5 Shooter]

I think I had this happen this weekend. Super humid outside here in Houston Texas this passed Saturday. Everything was sweating going from my 70f house to outside and humid. Gun. Bullets and especially me lol.

was testing my dasher load of 32.8g SW Precision running 109’s at 2890f/s. (2nd outing with SW Precision after finding a good node the first time with it) Velocities were right on with my load I found the first time with SW but man was I getting super sticky bolt lifts. Ejector swipes.

Was shooting 3 shot groups total of 30 (15 with br4 15 with 450’s) just check accuracy I had got the first outing with the SW and also checking sd/es with larger sample sizes.

The last 6-9 shots. No pressure. Im assuming the ammo and gun components acclimated to the conditions to stop what was happening. Barrel heating up Im sure helped.

Weird though as it was my first experience with this. Im assuming it was indeed the sweaty Bullets and components causing the pressure issue.

Gonna reload exactly the same this weekend. Bring a towel and wipe everything dry if it is sweaty and see if the same thing happens or not.