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Old June 4, 2010, 04:53 AM   #1
rauke
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Effect of heavy rain on bullet trajectory?

I’m just wondering if there are any bench rest shooters or hunters reading this who have experienced firing groups from the same rifle in good weather, then firing groups in heavy rain.

I’m just curious as to how much heavy rain might affect the trajectory of a .308 rifle bullet at say, 100 yards, granted that you could see that far in a rain storm.

(Right now, I’m thinking that a bullet hitting several raindrops on its way to the target is going to have its velocity reduced and its trajectory altered, however minimally. I would also imagine that heavier bullets would be affected less than light bullets. But this is just all conjecture on my part. You would think that the Army or the Marines would have definitive studies covering this. Unfortunately, I haven’t been able to turn up any thing so far.)

If you had a rifle that normally grouped one inch at 100 meters in good weather, does any one know how big that group might become in a heavy downpour (say, 2 inches of rain falling in an hour)?

I know that a lot of you have hunted in all kinds of weather, and I’m hoping you might share some of your practical observations, for the enlightenment of any one who might have to take a potshot at a possible trophy in weather fit only for ducks.
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Old June 4, 2010, 05:44 AM   #2
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Never have tried it, but the way I see it, if the rain is that heavy, you can't see a game animal far enough away for it to matter. Guessing, deflection might be a quarter-inch in a hundred yards?

A rainstorm of that intensity would have winds which would be the major problem, even if vision were not.

I've had thunderstorm rains of two inches in forty minutes at my house, so I'm basing it on observation...
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Old June 4, 2010, 07:50 AM   #3
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I've often wondered the same thing myself...and I tend to agree that heavy rain should have some negative effects on shooting, but so far my experience has been - If I can still see it clearly, I can still hit it. Not scientific at all, but I've made a few shots in heavy snowstorms and moderate rain.
Though as Art Eatman said, I could see wind being a bigger issue.
Around here the wind tends to swirl and blow in different directions even from one short distance to another. Perhaps I'm lucky, but on days like that, I've had good luck waiting for whatever bit of calm I can and then firing straight to target with no compensation for wind....definetly not the way to do it, if the wind is blowing in one direction or the other, but when it swirls its what I do, and its worked well so far, out to about 250 yards or so.
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Old June 4, 2010, 09:04 AM   #4
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I would guess that, somewhat counter to one's intuition, the probability of a collision between a bullet in flight and a rain drop is pretty low, even in a heavy rain.

On the other hand, rain usually means a high relative humidity level, which directly affects the density of the medium through which the bullet is traveling, which would be properly modelled by changing the drag coefficient in a ballistic program. However, my guess is that the effect in terms of printing on a downrange target is low enough to be entirely masked by other factors.
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Old June 4, 2010, 09:17 AM   #5
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Interesting question....

Tag for actual observations.
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Old June 4, 2010, 09:22 AM   #6
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I've certainly been out in conditions that would have affected bullet trajectory. I'm not thinking the rain itself would be the factor so much as the gusting winds.

Now I'm trying to think up a way you'd test this.
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Old June 4, 2010, 09:42 AM   #7
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You need to address you question to High Power Shooters.

Any one who has shot any HP Matches has shot the same rifle and ammo in Rain or Shine, Cold 'n Hot. And many times all that in the same match.

I believe a good example would be the 1978 WPW (National Guard Championships) at Camp Robinson, Little Rock AR.

The individual match started hot and clear, ended up with Little Rock having the worst flooding in 50 years. They had to cancil the Short Range Team Matches because of flooding in the pits.

Anyway, to answer your question, Rain Affects the shooter more then it affects the ammo. Some shooter's scores didnt change at all, some got worse. Even in the Rain, if you can see the target, you can hit it.

Where rain could effect a M14 or M1A is a drop of water lodging in the flash suppressor before you fire. That was addressed by the AMU when they first started "matching" the M14s, Hense, thats why a #7 reamer is used to open up the flash suppressor.

Snow is more fun, wait until you shoot in a white out, and wet snow completely covers the target where you cant see the black. When the bullet hits the target, all the wet snow slides down on the targer puller. Cant see the black bullseye, and you cant use a frame hold because there is white all around the target frame.

Anyway, rain/snow affects the shooter, not the bullet. Some can do it some cant. The problem is we (my self included) dont practice in a rain or snow storm, we set by the fire. They dont cancil HP matches for rain/snow, so we dont fair so well, and this is mistakenly blamed on rain affecting the bullet.

I'm an old lazy, wimpy, has been, I shoot for fun, not to prove anything now days, I dont have to MAKE THE TEAM, or keep my Coaching Job. So I'm gonna set by the fire instead of practicing in the rain and snow.

On a side note, when I did take my shooting serious, Heat bothered me more then rain. Burning sweat rolling into my eyes affected my scores more then any rain drops. But thats just me. Like guns, all shooters are differant.
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Old June 4, 2010, 10:29 AM   #8
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While with 308 FMJ the influence of rain on trajectory etc might be neglible, I'd like to see the result on lets say a 22-250 with a 32 gr varmint grenade. I'd be worried the only thing arriving at my target it dust.
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Old June 4, 2010, 11:05 AM   #9
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You did say 100 yards. The one club I belong to has a roof built over the benches and I often go when it is raining, less people there and it is the closest to my house. I can recall a few downpours when shooting and I kept on shooting, because it rains when you hunt anyway. I did not notice a bit of difference. The shooting lane I was on had two earth berms running the whole length so wind was no problem. The only thing is I know I never did that with less than a 140 grain 6.5 rifle. Like the other poster said, I really don't know what a 22-250 would do.
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Old June 4, 2010, 11:18 AM   #10
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Here in western WA we get to see a lot of rain. I still shoot, rain or not. My regular sight settings still work out to 600 yds. If it rains too hard, I am more concerned with water bridging the peep sight than the bullets hitting water drops. I have noticed that when I go to eastern WA (much drier) that my POI is slightly higher.
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Old June 4, 2010, 11:37 AM   #11
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At 100 yards wind yes, rain no unless its blowing in your eyes and effecting your vision. Same for snow and fog. If you can see the target you can hit it.

At 600 yards you might have a problem but at 100 your little bullet is still going to be hot and dry.

Barometer and humidity has more effect than rain does in my experience.
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Old June 4, 2010, 12:43 PM   #12
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I doubt you would have much deviation with a real man's round like the .308 in the rain but with some "rifles" (AR15) you should expect the .223 round to be wildly off course.
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Old June 4, 2010, 01:27 PM   #13
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I was also thinking the smaller bullets would be more affect by collision with raindrops. But it sounds like, from this thread, that people arent' seeing much affect.

Can anybody who has actually shot small caliber bullets in the rain at a target comment?
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Old June 4, 2010, 01:49 PM   #14
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I've wondered about this. Would the dense air surrounding a fast moving projectile actually let the rain hit the bullet? It seems to me that the atmospheric pressure with regards to storms would have more bearing,IMHO
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Old June 4, 2010, 04:49 PM   #15
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First off, I've never seen a difference between shooting in the rain, and shooting on dry days. The biggest issues I've had, are long shots during the middle of summer. The "heat waves" causing refraction can play hell with seeing your target.

Rain makes up 0.2% of air volume during a 2-inches-of-rain-per-hour storm. (With 0.016 gallons of water falling through a cubic foot {7.5 gallons} of air per second.) It makes up far less than that during normal rain storms.

You have a 1:500 chance of hitting water in a one cubic foot air space (2"/hour) during a 1 second time interval.

Remember - The human brain makes us see things that are not necessarily real. It may look like a wall of water, but it's a bunch of individual rain drops. The streaking is a processing effect, done by our brains, so we have time to "see" the rain - rather than the brain only acknowledging it subconsciously.

To look at the odds of hitting water, let's look at only the bullet's path.

Imagine the bullet will travel through a "cylinder" of air.
With a .308" bullet, we need to find out how long this cylinder will be, to have a volume of one cubic foot.

1 Cubic Foot = 1,728 Cubic Inches
The volume of a cylinder is calculated as:
Pi*radius^2 (of circle)*length (of cylinder)

The radius of a .308" bullet is .154".
Pi(3.141)*.154^2 = 0.234 square inches.

1,728 cubic inches / 0.234 sq in = 7,382.5 inches

So, a .308" diameter cylinder needs to be 7,382.5 inches long to contain one cubic foot of volume. (7,382.5 inches / 12 inches per foot = 615.2 feet)

Which means.... your .308" bullet can travel 615 feet or 205 yards with a 1:500 chance of hitting water. Move closer to the target, at 100 yards; and you have a 1:1025 (0.0975%) chance of hitting water.

If it's only raining 1"/hour ... those odds go to 1:2050 (0.0488%) at 100 yards.

And that's not even figuring the adjustment for bullet flight time. Most bullets won't take a full second to reach the target. Therefore, they won't have a full second's worth of exposure, and the odds of hitting water are decreased even further.

For example (simple calculation with constant velocity):
A 2,100 fps bullet will reach a 100 yard target in 1/7 of a second (2,100 feet per second/300 feet = 1/7 second).
That means the odds of hitting water drop to a 1:7175 (0.0139%) chance of hitting water.

The odds are in your favor.

*Some of these numbers were rounded to certain decimal places.
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Last edited by FrankenMauser; June 4, 2010 at 05:24 PM.
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Old June 4, 2010, 04:49 PM   #16
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If the bullet actually hits the rain drop it's like hitting a little pebble. At those speed water is "hard". Bullet speed is kind of "in between" for me, but I've seen what rain does to a Mach 6 target, and it's not pretty, looks like sandblasted with a very coarse grit.
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Old June 4, 2010, 04:51 PM   #17
kraigwy
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Quote:
but with some "rifles" (AR15) you should expect the .223 round to be wildly off course.
You can "expect it" but in reality it dosnt happen.
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Old June 4, 2010, 06:25 PM   #18
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A high-velocity bullet has a shock wave ahead of it (supersonic) that may fracture a raindrop. I haven't heard of any deflection caused by rain.

I've observed 44 mag handgun bullets in a rainstorm through a spotting scope and they do create a mist trail in heavy rain, but accuracy wasn't precise enough to tell whether there was any deflection.

A few years ago, we held a rimfire benchrest match at 50 yards and saw mist from bullets hitting raindrops though the 36x scope, but didn't notice any deflection. Scores were normal.
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Old June 4, 2010, 06:43 PM   #19
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I am not sure about what effects rain has on bullets, but I must say this is the most original question I have seen in awhile.....
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Old June 4, 2010, 06:49 PM   #20
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I don't know about bullets, but the rain had no negative effect on the charcoal grilled lamb chops I just threw down my throat for dinner! Deeelish!!
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Old June 4, 2010, 07:35 PM   #21
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In 2007 I was shooting in the NRA Championships at 600 yards on Viale Range. I was shooting a .223 AR match rifle.

The weather was overcast and the skies alternated between dry and rain with an evil fickleness. First relay, which I was on, the line was prevented from going hot by interminable stupid problems and poor officiating. It took forever to get to the preparation period, and once having completed that, the whole line was ordered to ground our rifles and stay in place. I saw a HMMWV roll from behind the range tower and trundle its way to the pits. Apparently someone in the pits had a dead battery and we all had to wait.

As we waited we could see this dark, ominous line coming in from the lake. After having taken enough time for what must have been a coffee and a donut, the HMMVW trundled back to its place behind the tower and the line was given permission to shoot.

I shot as fast as I could, knowing that I was going to be pasted with rain. Probably less than 15 seconds per shot, so good was my pit service, and I fired as the target came up. But so it was, before I got to shot 10 the rain squalls hit. It rained so heavy that I was blowing water out of my rear aperture. My shots were blown left and right by the wind gales, and there were 8's, but I did not change my elevation. There was this period of no wind but heavy rain, and I was hitting X’s, even though I could not see the bull. I was centering the frame in my aperture.

My score was ruined, my databook so wet I could not write in it, my scope fogged up. I and all of my equipment were saturated with water.

The USMC Marine Shooter, with whom I was squadded, had an equally awful time at long range. When we went to the pits, he being in his vigorous 20's, covered his equipment on the line with his poncho, and let the rain pour over him as he pulled targets.

I am of the conclusion that rain does not have that much of an effect on the path of bullets, not nearly as much as wind. Or course, if you can’t see what you are doing, maybe you should not be shooting anyway.
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Old June 5, 2010, 09:19 AM   #22
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Just shoot between the rain drops.
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Old June 5, 2010, 09:49 AM   #23
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+1 on Picher's comment.

There's what's known as a bow shockwave that's propagated surrounding the bullet in flight. This is going to disrupt any incoming rain drops.




This question comes up every once in a while on HP and long range forums. It's easy to assume that there must be some affect, because you envision the bullet crashing into rain drops. But that doesn't seem to be the case, and anecdotally there doesn't seem to be any affect from rain, its the change in atmospheric pressure, and wind that are the culprit.
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Old June 5, 2010, 12:41 PM   #24
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Quote:
There's what's known as a bow shockwave that's propagated surrounding the bullet in flight. This is going to disrupt any incoming rain drops.
Every action has an equal, and opposite, reaction.

A rain drop in the bullet's path will change the pressure waves, and the bullet's flight. It may not be detectable without extremely precise instruments, but the effect is still there.
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Old June 5, 2010, 11:06 PM   #25
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Frankenmauser,

Quote:
It may not be detectable without extremely precise instruments, but the effect is still there.
If the effect is masked by normal "noise" then the effect is insignificant. Basically if the shot group doesn't wander from the fair weather norm, then rain has no effect. You have to have a measurable data point outside of the normal chaos in order to establish a cause/effect relationship.

No I agree, if a fly lands on the bow of the USS Nimitz the ship does deflect slightly. We can't measure it because the normal waive action along the surface line of the hull has a range that will mask any insignificant event. From the perspective of the engineering mate three decks down, the fly does not exist.

All rain does is make you account for 100% humidity on your ballistics computer. For our purposes the rain does not exist as a ballistic influence. .

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