Discussion in 'Rifle Country' started by grogetr, Nov 15, 2015.
Walkalong, I"m not the best shot around I have trouble holding still so yes I think in the right hands the groups would be better but I will keep practicing and hopefully get more comfortable.
A wind meter & ballistics app together will get you close but because of longer distance & manipulation of wind direction by the terrain in between you & target .only practice will help you conquer the wind
Good rule of thumb if you don't have a ballistic app at hand is 1moa per 100yd per 10mph of crosswind.
Theoretically, I understand why a heavy for caliber bullet slows down less than a light for caliber bullet; the drag force only depends on the frontal (cross sectional) area of the bullet (which doesn't change if the caliber stays the same). As the bullet gets longer its mass increases, so the same drag force is operating on a larger mass and thus slows it down (negative acceleration) at a smaller rate.
When I think about wind hitting the side of the bullet, if a heavy for caliber bullet is 1.75 times the mass of a light for caliber bullet, it's about 1.75 times as long, and thus the wind has 1.75 times the area to apply force -- it seems like the light and heavy bullets would have the same wind drift if they had the same time of flight. To me, intuitively, it seems that the lighter bullet would have less wind drift in cases of all but the longest range shots (because the lighter bullet generally reaches its target faster).
Also, if you are a very serious shooter, is the g1 drag coefficient a useful tool for calculating wind drift, or do you use some other coefficient? When I look at the bullet that the g1 coefficient was based on and compare it to a heavy for caliber 223 bullet it's like comparing a cannon ball to a canoe. I understand why g1 would be useful for calculating drop (because you adjust g1 until it fits the drop you observe), but it seems that g1 would underpredict wind drift in any bullet you would shoot from a shoulder mounted weapon.
Thanks everyone, I learn a lot on this board and appreciate your input. There aren't many long range shots (or much wind) where I hunt, so I've never had to learn how to shoot in the wind.
Interesting question. The ballistic calcs consistently say that heavy-for-caliber loads have not only better energy retention due to BC, but better wind performance at least in the calibers I shoot. But now that you bring it up, I don't really understand why.
A given crosswind speed close to the rifle has more effect on downrange bullet drift than that same crosswind near the target.
Wind above the line of sight is faster than in the line of sight; how much depends on the terrain. Smooth, open, flat land has lower cross winds at the bullet's maximum ordinate than terrain with low bushes and short trees.
This is not an accurate statement. Wind at the target has more effect on a bullet than wind closer to the rifle's muzzle. As the bullet flies, it loses speed and is more susceptible to bullet drift closer to the target where it has less speed to cut through the wind.
i can't think of a way to do that in something like jbm
223 is going to be even worse with those low bc numbers.
I was going to mention this as well. That weight is part of the ballistic coefficient determination so more weight leaves you with a higher bc. Bc is really a combination of factors you would normally have to enter one by one but the manufacturer is usually nice enough to just give you a number to plug in with everything figured out.
But, the bullet doesn't stop drifting when the wind stops. If you have zero cross wind from 0-150 yards and a 10 mph cross wind from 150-300 yards, the bullet is only drifting for 150 yards. If there's a 10 mph cross wind from 0-150 yards the bullet drifts, and even if there's no wind from 150-300 yards the bullet will continue to drift because it's already moving in that direction.
If you setup the problem with a 10 mph right to left wind from 0-150 yards and a 10 mph left to right wind from 150-300 yards then the answer is less clear, i can run som back of the envelope calculations when I have some time. Intuitively I'd side with Bart, but it's not obvious like it is in the former case.
Actually, Bart is indeed correct.
using a 168g SMK with 2650 fps MV (defaults for all other values in jbm)
at 500 yards, the bullet has 26" of wind drift and is now only traveling 1717.8 fps
if we then stop and recalculate based on a MV of 1717.8 fps
at 500 yards (which would be 1000 yards if we added them together) we get 52.7" of wind drift and a MV of 1040 fps
so the second 500 yards has almost exactly 2x as much drift as the first 500
HOWEVER that doesn't take into account at least two issues and maybe more:
1. max ordinate is going to be in the second half of the trajectory, and wind is typically much stronger way up in the air than it is at the muzzle.
2. this is simplistic in that in the second calculation, it doesn't take into account the direction of travel of the bullet, which is not toward the target, but at an angle based on the wind from the first 500 yards
The variable is how much wind where and what direction. But for a reasonably constant wind the effect nearer the muzzle is now magnified by any other movement farther downrange, not to mention the fact that going offline early is magnified more and more as it moves away from that spot.
If you are shooting 500 yards and the wind is light to two hundred, medium to 400, and heavy to 500, then yes, it gets moved more further out.
Shooting 200 yard Benchrest you would see lots of flags at 10 yards to 150ish yards, but rarely any further out. I never saw any farther out than 175, but everyone had one closer than 25 yards to the muzzle. I would put out four flags shooting at 100 yards and five flags shooting at 200 yards. Only one more.
If you use good ballistic software to see how many inches sideways per 10 yards of downrange travel the bullet is moving when the wind stops, you can easily calculate how much to the side it will strike in the wind direction when it finally gets to the target.
Here's Sierra Bullets' software plotting wind drift through 1000 yards. Green line's the bullet's horizontal path with a 9 o'clock 10 mph cross wind for the first 333 yards; red line for the last 333 yards.
This assumes the wind speed is constant at all points in the bullet's trajectory. No ballistic software I know of adjusts for wind speed above the line of sight that varies with terrain.
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