WHY doesn't rain cause point of impact to change?

U

Unlicensed Dremel

Moderator
It's generally well-accepted (and proven to my satisfaction personally), that the POI from a rifle or other gun doesn't change materially when it's raining, than when not.

But how on earth can this be? How could it be that a bullet going over twice the speed of sound, impacting an object (a raindrop) which has a mass of anywhere from a fraction of the bullet's, to almost as much as the bullet, in the case of large raindrops and/or light bullets, travelling downward at terminal velocity, thus with significant momentum - NOT severely change a bullets course? And not just once, but surely there are several collisions with raindrops on the way to the target, depending on rain density and distance to target. It blows my mind. I would expect to see massive differences (unpredictable from shot to shot, but generally downward) in POI from a heavy rain, and significant ones even in a light rain, especially from small, fast bullets, or small slow bullets too.
 
The best thing I can think of is that the supersonic pressure wave riding ahead of the bullet disperses the drops before contact with the projectile. Subsonic I have no idea.

Empirical evidence. (From way back before this was thought of as a bad thing:rolleyes:)

We were shooting .30 cal military ammunition & needed to waste some. The range was 300 yds but at 150 or so there was a glass milk bottle floating open end down in a big puddle. It was about 2/3 submerged. We shot lots of ammunition at that & it never broke. The spotter said it was flipping violently from side to side every time someone fired & that's where the idea comes from.
 
What does a rain drop weigh?
Raindrops range between 2 and 6mm diameter
1cc water weighs 1 gram; so an average raindrop of 2mm diameter would weigh 0.042gm (.65grain) while a 6mm diameter drop would weigh 1.13gm (17.5 grain)

How much rain is there per cubic foot?
USGS says between 50ea (2mm/3mm) [medium/heavy rain] to 113ea (4mm) [cloudburst] drops per second per cubic foot (1,728cu in)

How much "bullet per cubic foot"?
Assuming (and wildly incorrectly at that) that a bullet .25" in diameter is going to hit a target 300yards (10,800") away, that's a cylinder of 530 cubic inches.

Top of the head, there's only a 1 in 3 chance of hitting a rain drop if the bullet takes a full second to travel 1200ft. Bump the velocity up to 2400sps and it looks like about 1 chance in 6 of hitting a raindrop.

Then again, math isn't my strong suit and I'm probably wrong by an order of magnitude at least.:cool:
 
The answer is that bullets rarely hit raindrops. If we take the figure of 113 drops per cubic foot per second, a bullet will pass through that cube in about 1/2500 of a second, so there is only 113/2500 chance of a raindrop even being in the "box" when the bullet goes through.

And even if all those 113 drops waited around for a whole second, there would still be a lot of that cubic foot that would not be water. (Think a box 12"x12"x12" with 113 small marbles in it. They wouldn't even cover much of the bottom, let alone fill up the space if rattling around.)

So bullets are not deflected by hitting a raindrop because they just don't hit a raindrop except on an odd chance. So some of us will have to find a better excuse for missing that big buck. "My bullet hit a raindrop" won't cut it.

Jim
 
A while ago, we had some similar discussions.
The one that seemed (to me) to have the best answers was this one: Effect of Heavy Rain on Bullet Trajectory.

Key points:
A 2,100 fps .308" projectile has a 1:7175 (0.0139%) chance of hitting a rain drop in a one-inch-per-hour rain storm, when fired at a target 100 yards away.

Even if the bullet did "hit" the rain drop, the bow wave coming off the bullet is likely to disperse/disrupt the droplet without it having any measurable affect on the bullet's flight.
 
Seems to me like one could try and make some awesome footage of bullets in slow motion during a downpour.

I would imagine rain wouldn't cause the POI to drop but perhaps make the groups open up at longer distances.

Now a hailstorm, that might be more susceptible to altering POI.
 
I cannot provide a link, but this question came up at some time in the past on a different forum. I forget the details, but there were enough links provided to convince me that the bullet never hits the rain. Something about pressure waves deflecting any rain and preventing it from ever striking the bullet. Hopefully someone else can provide the information.
 
Unlicensed Dremel said:
WHY doesn't rain cause point of impact to change?
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Simply put, because hitting one is like winning the lottery.

The math is fairly simple. Consider that you might have a 5" per hour rainstorm and clouds at 10,000 feet. Figure the volume of an average raindrop and then figure how many of those it takes to cover a bullet width of ground between your gun and target with 5" of water. Divide the volume of the air between the ground and clouds by the volume of those rain drops and that's the percentage of air that's filled with a water drop DURING THE ENTIRE HOUR of a 5" per hour storm. Divide THAT number by the amount of time that your bullet takes to get to the target and you've got how much water is in the air while your bullet is traveling.

Or, just read this post.:D

It's pretty small.
 
Now a hailstorm, that might be more susceptible to altering POI.
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I suppose its possible, but hail, like ice and snow, is lighter than water, per given area. So balance the slightly increased chance of a bullet hitting hail (the big ones) due to their larger surface area, vs. how many of them are in the volume of air the bullet passes through WHEN the bullet is passing through.

Rain affects your sighting, more than it affects the bullets travel.
 
Rain does not cause a POI change because a bullet traveling at supersonic speed never actually hits a raindrop. When a bullet flies at supersonic speed, there is a layer of compressed air at the tip which disperses evenly across the bullet. You can see this in high-speed photos of supersonic bullets. As the layer of compressed air touches a raindrop, it disperses it evenly around the bullet, the bullet never actually touches a raindrop because it is shielded by compressed air.
 
The best thing I can think of is that the supersonic pressure wave riding ahead of the bullet disperses the drops before contact with the projectile. Subsonic I have no idea.
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Which is exactly what a physicist told me years ago. The surrounding shock wave around the bullet disrupts the path of the drop until velocities drop below sub-sonic. (Or so I was told.)
 
QUOTE: You can see this in high-speed photos of supersonic bullets. As the layer of compressed air touches a raindrop, it disperses it evenly around the bullet, the bullet never actually touches a raindrop because it is shielded by compressed air. QUOTE
__________________

:cool: Just like the Starship USS Enterprise creates a warp bubble as it travels through hyperspace :eek:
 
Most folks don't shoot in the rain because it's unpleasant but if you talk to folks who do (competitions don't usually end because of a little rain), you'll find folks who have seen bullets hit raindrops.

I don't believe that the shockwave prevents it. If the shockwave deflects the rain, it also deflects the bullet. Action/reaction. Force/opposite-force. From a visual perspective, the raindrop would "splatter", whether it was directly hitting the bullet or shockwave. You would (and can) see of little "POOF!" of water when a bullet hits a raindrop.
 
I've shot a few long range matches in the rain using .308's and 30 caliber magnums. And once in a hail storm bouncing pea sized hailstones off my barrel (I could hear them do that off adjacent barrels, too). Never saw any shots going off call any more than the normal 1/4 to 1/2 MOA.

Watching the atmospheric trace of another shooter's bullets going to targets 200 to 1000 yards away through the rain; none seemed to change paths and all flew as straight and normal as they're observed in clear weather.
 
So, if the supersonic pressure wave pushes water out of the way, what about subsonic rounds? There are situations where a subsonic bullet will be shot at long ranges. The 9x39 as a sniper round was designed to be effective past 500m as a subsonic load. The slower moving bullet has a much greater chance of contacting rain drops.
 
And once in a hail storm bouncing pea sized hailstones off my barrel (I could hear them do that off adjacent barrels, too). Never saw any shots going off call any more than the normal 1/4 to 1/2 MOA.
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See!?! Isn't that weird?
 
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