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Old September 3, 2013, 12:08 PM   #26
Brian Pfleuger
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There's not nearly as much water in the air during a rain storm as some folks seem to believe.

Let's just use a simple thought process as an example. It may not be perfectly scientifically, mathematically correct but it's close enough to give you an idea.

Let's say it's raining at a very heavy 5 inches per hour. This is at least 20% higher than, and more likely 2-5x higher than, an average rain fall.

To get a really good number and put everybody to sleep, we'd have to use some fun calculus (Δt → 0) and other fun stuff.

I'm going to guesstiproximate instead.

Let's go with a 180gr .308 bullet at 2,600 fps MV. That bullet (according to JBM Ballistics) would take about 1.9 seconds to get to 1,000 yards. Let's call it 2 seconds for easy math.

Let's do some simple extrapolation,

A 5"/hour rain fall literally means that it takes one hour to fill a space with 5" water. There are 3,600 seconds in an hour. The bullet is in flight to 1,000 yards for 2 seconds. During that 2 seconds, only 2/3600ths of the total 1 hour rain will fall. That means that during the entire bullet flight time, the total amount of water in the air is 5 inches x 2 divided by 3,600 seconds. That's 1/360th of an inch (~0.0027777) of rain in that time. That amount is broken up evenly throughout the entire space.

Let's consider the space taken up by the bullet. Let's make it a 1,000 yard cylinder, 0.308 inches diameter. We have a cylinder 3,000 feet (36,000 inches) long with a .308" diameter. The total volume of that cylinder is 2,682 cubic inches. An average raindrop might be about 0.078 inches in diameter. That's a volume of ~0.0019877 in3.

In that 2 second flight time, we're only filling that entire cylinder with 1/360th of an inch of rain. That's a total volume of rain within that cylinder during time of flight of 0.000206955 in3.

Look at those two numbers, total volume and rain drop volume.... there's more zeros in the total volume than there are in a single rain drop!

That means that in the entire flight of the bullet, there will be less than one rain drop in it's entire path... ahead AND behind it!

Not just "less than one", a single drop of rain is almost 10 times the total volume of water in that column during the entire bullets flight!

You tell me. There's essentially 1 drop of water SOMEWHERE in the flight path of every 10th bullet. That's in front and behind. We could sort of almost consider that it would take 20 shots to get a single drop of rain to be somewhere in front of the bullet during it's time of flight.

You wanna take the bet that you're going to hit one?
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Old September 3, 2013, 04:11 PM   #27
johnwilliamson062
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Quote:
That means that in the entire flight of the bullet, there will be less than one rain drop in it's entire path
Now that is good statistical analysis!

Quote:
There's essentially 1 drop of water SOMEWHERE in the flight path of every 10th bullet.
From how I read your math on getting the original raindrop chance I think you calculate the chance a raindrop would be entirely in the same spot as the bullet at any point in that 2 seconds. As in 1 of ten bullets will hit a rain drop. Realistically they aren't going to hit an entire raindrop. Lets say 1 in 20 hits an entire rain drop, 1 in 20 hits 1/2 a rain drop, 1 in 20 hits 1/4 of a rain drop. Just a hypothetical breakdown, but it goes to show that more than 1 in ten will hit some sort of water.

I'm still not worried about the waters effect though.
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Old September 3, 2013, 04:42 PM   #28
Brian Pfleuger
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The math isn't technically valid, it's just for an example of the probability, but I'd point out that it doesn't suggest that 1 in 10 bullets would contact a drop. It suggests that only one in 10 bullets would even have water IN IT'S FLIGHT PATH, in front or behind it, during ANY portion of it's 2 second flight.

If that's even remotely close to true, even if it's pessimistic by a factor of 100, the odds of a bullet actually HITTING a drop of water are exceedingly small.

They odds that they'd both be in the flight path ANYWHERE are about 1 in 10. The locations have to over-lap, not just both be somewhere in the flight path.

For example, the bullet takes up roughly 1/36,000th of the flight path. The drop of water takes up about 1/461000th of the flight path. The odds that they'd randomly be in the same place at the same time would be something like, 1:357000 or something.

Someone with more time will have to figure it if they'd like, I'm trying to do it between customers and pizza.
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Old September 3, 2013, 05:27 PM   #29
Dashunde
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Density and heat...
A 2700fps 168gr jacketed .308 wouldn’t be the lest bit phased by a raindrop hit broadside.
The raindrop perishes into water vapor - mostly converted to gas due to the heat.
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Old September 3, 2013, 05:33 PM   #30
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How much does rain affect accuracy?

Quote:
Originally Posted by JD0x0 View Post
I would have to argue against that. The VSS sniper rifle had an effective range of 500-600 meters with a 9x39 subsonic bullet. A .338 whisper with a 300 grain boat tail has about the same amount of drop and time of flight at 500 yards that a .30-06 has at 1000 yards. With only about 16 inches of wind drift in a 10mph 90 degree crosswind, and has only lost about 120fps of velocity at that range.
A .338 Whisper has only lost 120 FPS after 500 yards? I highly doubt it.
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Old September 3, 2013, 07:07 PM   #31
JD0x0
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Quote:
A .338 Whisper has only lost 120 FPS after 500 yards? I highly doubt it.
Punch in the numbers in a ballistic calculator if you don't believe me. Remember that wind resistance grows exponentially with speed. So slower bullets generally lose less velocity over a given range, compared to the same bullet at a faster speed.

For the example I posted I used a calculated G1 BC of .734 for a 300 grain .338
Berger Bullets 300 grain hunting bullet has a listed G1 BC of .818

I used a temp of 65 degrees at sea level. A subsonic muzzle velocity of 1050fps. Here's what I came up with, with the lower BC of .734

Quote:
Range: 0
Velocity:1050

Range:100
Velocity: 1021

Range:300
Velocity:975

Range: 500 yards
Velocity: 938
With everything else constant, using Berger's BC of .818
Quote:
Range:100
Velocity:1025

Range:300
Velocity:982

Range:500
Velocity:948
Feel free to try yourself, if you don't believe the numbers I posted. I used Handloads.com Ballistic calculator FWIW.
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Old September 3, 2013, 08:26 PM   #32
Vanya
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Quote:
Originally Posted by Dashunde
A 2700fps 168gr jacketed .308 wouldn’t be the lest bit phased by a raindrop hit broadside.
The raindrop perishes into water vapor - mostly converted to gas due to the heat.
But the raindrop is converted to vapor in a tiny explosion that will push on the bullet and deflect it ever so slightly, unless the bullet hits the raindrop absolutely dead center. The size of the explosion will depend on that of the raindrop, and on the temperature of the bullet. The result of the deflection will depend on the relative distance of the bullet between the muzzle of the gun and the target: the closer to the muzzle the "raindrop event" occurs, the farther off-target it will push the bullet.

All that said, I suspect that the actual effect would be, um, negligible.
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Old September 3, 2013, 08:47 PM   #33
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I am not sure about the math, but I can tell you from personal experience that rain has a terrible effect on accuracy. It keeps wimpy people from practicing!

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Old September 3, 2013, 08:57 PM   #34
johnwilliamson062
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Quote:
That means that during the entire bullet flight time, the total amount of water in the air is 5 inches x 2 divided by 3,600 seconds
That is the calculation for a unit of space, not the entire flight path. When I read your calculations it seems to me you calculated the entire length of the "tube" would get 5" of rain, not every square inch. At the end of an hour we have a .308 wide 5" high mess of rain, not a tube with 5 cubic inches of rain poured into it. 5 inch per hour is more or less a hurricane BTW.

Quote:
In that 2 second flight time, we're only filling that entire cylinder with 1/360th of an inch of rain
I think in two seconds we are filling every point along the tube with a vertical 1/360th of rain. Along the complete length of the tube for two seconds something like .308X1000X36X2/360 would fall. 61.6 in^3 of rain. Over the course of an hour .308X1000X5, or 55,440 in^3. In two seconds that would be .308*1000*36*2/360/.0019877 or almost 31,000 rain drops along that path. I think in such a torrential down pour the diameter of the rain drop would be at the high end near .13. That results in a rain drop volume of .009in^3 and a total raindrop count for two seconds at about 6,844 along our bullets path. ~3,5drops/second in an are .308X1 yard. About 12 square inches. 18 drops per second on a human standing upright. Having been in several hurricanes I am confident this is a reasonable number. Maybe with all the estimates it is double the correct value. With your calculation one rain drop falls on 77 square(the area of our 1000 yard X .308 flight path) feet every two seconds during a 5in/hour rainfall. That doesn't pass the smell test for me.

I'm not 100% sure, but that is what I get from reading your calculations and developing my own method(I admit based off your method where the heavy lifting was accomplished). There was a time when this sort of problem would have been extremely simple for me, but all I deal with now is simple statistics and the word problem analytic training has faded. I might be off base in my thinking.

Where the bullet is located or whether it is moving is actually irrelevant. In theory the chance that an object with the area of the bullet will get hit by a raindrop is the same whether the bullet is moving or still, at least in our simplified algebraic model. I'd have to consider what adding the bullet flying into raindrops instead of them just landing on top would do, but I am quite certain that would increase the likelihood of contact, not decrease.
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Old September 6, 2013, 07:33 PM   #35
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I have noticed a very small increase in group size when raining. Maybe 20%? It is consistent but I think it is due to the humidity more thAn anything else. A little increased pressure at or near the barrel that varies with the strength of the rain.

But this is just an educated guess and I am an engineer.
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Old November 1, 2013, 12:55 PM   #36
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I remember when we shot IMHSA handgun silhouette and several of us were shooting .44 magnums with cast, gas-checked SWC bullets. A spotter behind the shooter with a 36x spotting scope could see bullets going to the 200 meter rams when the sun was behind us. He could also see the bullets hitting raindrops, (or having the shock wave hit them) in a heavy rainstorm. There didn't appear to be any measurable effect on the bullets from the rain, but we didn't shoot especially accurately, say, 2 MOA.
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Old November 1, 2013, 07:29 PM   #37
Bart B.
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I've shot a few matches in the rain. No significant accuracy issues through 1000 yards in my opinion. But lots of folks blamed bad shots on raindrops.

Biggest problem I've heard of regarding accuracy was when the M14NM rifles first came out. Their flash supressors were small enough that rain drops on their inner surfaces deflected bullets enough to lose accuracy. After the services reamed them out, the problem went away shooting in the rain.

But there were other problems with bullets barly skimming supressors on their way out, so the NM version was reamed out to allow clearance for that, to. There was even a gauge made to see if the supressor had enough clearance.
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Old November 25, 2013, 05:09 PM   #38
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the only difference i can see where i live is that the humidity of the air would be greater than normal. I live in Az and on a humid day we have 15%-20% humidity so shooting in the rain (rarely happens here anyways) would cause the zero to be off due to the heavy air slowing the bullet down. a good friend had this issue when he went for a sniper school back east, he zeroed his rifle before he left in about 5% humidity and then when he got to 95% humidity he had to re-zero all over again due to it being way off.
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Old November 30, 2013, 09:31 PM   #39
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I have shot out to 600 yards in the rain and yes, when it is raining, my scores went down.

One assignable cause was water on the lenses of my shooting glasses, the muddy ground, the mat slick , the rifle being slick, and of course, wind.

Sometimes when it rains, it blows. This is obvious when you are in rainy weather. Nasty gusting winds, such as I have shot in at Camp Perry, blow bullets all over the place. You can actually see circular wind patterns making their way through the grass. One range, we had to stand on our shooting mats because they would have blown away as the front came through, and I recall seeing one of those cheap styrofoam coolers tumbling down the range and flying over the berm.

Shooting a good standing score becomes much harder when wind gusts are high enough to blow your scope stand over.

I have shot smallbore in a drizzling rain. We managed to have pop up's so we shot mostly under cover, but I don't recall scores being especially stellar. You cannot see the target as well, because there is all that rain between my eye and target. This is obvious when you are actually there.

I have never found that range where you can shoot in the rain without myself, or my equipment getting wet, and where there is no wind. Perhaps when I find a place with “dry rain” and no wind, I will find that my scores are not all that different from nice sunny 70 F weather.

That will probably be in an alternate plane of existence.
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Last edited by Slamfire; November 30, 2013 at 09:38 PM.
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