Effects of Rain on Bullet Trajectory

I’ve heard a fair bit of conjecture over the years about what happens when a bullet flies through the rain. Speculation varies from nothing to maybe something. Until now, all I’ve had is an opinion. After conducting tests at EMRTC (New Mexico Tech) (emrtc.nmt.edu) for Guns & Ammo TV, observed by a team of ballisticians, I can honestly answer, “A lot more than you think.”

The most common speculation I’ve heard is that a pressure wave forms on the supersonic bullet’s nose and pushes anything out of the way — and water never touches the bullet. I’ve also heard that even if water hits the bullet, it moves too fast and has enough mass that a drop has no effect. Both hypotheses are false.

The impetus for this test was Dave Emary who write’s this magazine’s “Bullet Board” column. Emary shared with me an experience he had shooting a High Power Match at Camp Perry, Ohio, during which some serious rain showed up. Dave was shooting and doing just fine until he had one bullet barely clip the edge of the entire 600-­yard target board. It wasn’t just a few inches away from the rest of his group — it was a few feet! Dave was shooting an M1A chambered in .308 Winchester using 180-­grain bullets when his bullet hit about 3 feet away from his expected impact.

We all might pull a shot now and then, but a guy like Emary shoots a lot and knows when he did or didn’t make a mistake. He knew right away that much error didn’t come entirely from him and has wondered about it ever since.

Fast forward a few years and Emary now works at EMRTC where he and I discussed ideas for TV segments. He was pretty sure that EMRTC had the instrumentation to film a bullet hitting a drop of rain, so we decided it was time to find out exactly what happens when a bullet hits a drop by filming it occur in high speed.

The tools to make this happen involved a lot of do-­it-­yourself construction. Imagine a large sawhorse laid longitudinally down the line of fire. A small PVC pipe hung under the sawhorse with holes drilled every inch or so its entire length. Hook a hose up to the PVC pipe and dial up enough water pressure to generate a steady drip along the entire PVC length and you have a simulated rainstorm under which to shoot. We didn’t even have to get wet.

The camera setup was specialized, called the “Schlieren technique,” detecting differences in air density. It also films at about 70,000 frames a second, so it can catch everything that’s happening in tremendous detail.

Our test rifle was chambered in .308 Winchester and we used Hornady 125-­grain GMX bullets because we had a bunch on hand. The camera was set to film a few inches in front of the muzzle when we booted everything up and started shooting.

I figured it would take us at least an hour to catch a bullet hitting a raindrop. Our artificial “rain” device was putting out a good flow of drops, about what I see when a real downpour hits the southern states. We had the advantage of lining up our raindrops along the bullet’s flight path, but I thought that would only help a little. We filmed a bullet hitting a raindrop the third time we fired the gun, so it proved far easier than I suspected. Up until this experiment, if you had asked me my opinion on the effect a drop of rain would have on a bullet in flight, I’d have said, “Not much.” Reality proved different.

Engineer David Hibbert and a couple of PhD candidates determined that a drop of rain induced a 3.2-­degree yaw on our 125-­grain bullet. Thanks to some judicious pixel counting by our big-­brained team of scientists, we also determined that the bullet’s yaw was not directly correlated to the flight path; 3.2 degrees is some serious yaw. We observed 4 inches of deflection at 50 yards, however, the bullet could have hit multiple water droplets due to our test setup.

That’s a big shift and one I’ve never seen in real life, even after spending several years shooting in adverse conditions. Unfortunately, the U.S. Army’s Special Forces doesn’t believe in inclement weather plans with regards to range time, so Uncle Sugar made sure I spent time shooting in extreme weather. I’ve never shot in a hurricane, but I’ve done plenty in a downpour. I’ve never seen anything as pronounced as this experiment.

The reason why we saw such a huge shift in the bullet’s flight is because our bullet hit a large drop of water shortly after leaving the muzzle. The drop also hit the top half of the bullet, so it was able to exert maximum influence on the bullet’s flight.

This experiment busted a couple of common myths. The first casualty was the “rain never touches the bullet” myth. It most certainly does. Our bullet had a clearly visible pressure wave formed on the nose, and the rain drop cut right through it without deforming. Only when the round drop touched the bullet’s ogive did it rapidly fragment.

We also saw a shockwave form off the disintegrating raindrop. That’s how hard the bullet hit it. The wave coming from the raindrop collided with the shockwave from the bullet’s nose and caused it to alter shape. Any disturbance to the pressure wave from the bullet’s nose will cause some deviation in flight and that, too, was caught on camera. As the two waves mixed, the bullet’s shockwave undulated, making it less stable and created more drag.

Does this mean that shooters need to worry about shooting in the rain? No. Even in a pretty steady rain, the likelihood of hitting a drop with a bullet is pretty low. However, there are a couple of instances where it would be worth remembering this study, should you find yourself shooting in wet weather.

The first is with any type of competitive rifle shooting. These types of events, depending on the venue and the competitor, can be high stress. That’s why they are the best way to learn and improve quickly. The high stress can cause competitors to look for problems when there are none. I can’t tell you how often I’ve seen a shooter adjust zero in the worst possible lighting conditions, instead of leaving everything alone.

The same goes for shooting in the rain. If a guy shoots and the bullet just clips the edge of the target board, don’t panic. It’s likely that the bullet hit a drop of rain. Just keep doing what you’re doing. Nothing is broken on the rifle or scope. Mother Nature just decided to intervene.

The other instance is long-­range hunting. Regardless of an individual’s feelings on the subject, this type of hunting is becoming more popular. It is also the same type of shooting most vulnerable to the influence of rain and yielding the most catastrophic results. I recommend avoiding long shots in a steady rain. Any impact will alter the bullet’s flight enough that a wounding shot is far more likely than in clear conditions. No animal deserves such treatment.

However, lots of guys have been shooting in wet weather for a long time and just about all of us have never had an issue. That trend will continue. Should you witness an erratic impact when shooting in a downpour, consider yourself one of the lucky few that’s actually shot a raindrop. After reading this column you’ve even got some evidence to share when you discuss the phenomenon with your friends. 

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