Wind, caliber, bullet weight, and ballistic coefficient

[PLhunter]

Your theory about the SD of the bullet in the wind is also flawed. As soon as the bullet exits the muzzle and experiences an asymmetric center of pressure due to wind from one side but not the other, its rotational (spin) axis rotates to coincide with the COP and the drag axis. The result of this rotation of the spin axis creates a torque applied to the axis that results in aerodynamic jump.

The actual SD is the effective and practical SD.

Tip turns into the wind slightly? Correct? In order for the functional sd to remain the same that would have to be the case if I’m thinking of it correctly now?

 

[Jordan Smith]

Tip turns into the wind slightly? Correct? In order for the functional sd to remain the same that would have to be the case if I’m thinking of it correctly now?

Yes, sir.

 

[PLhunter]

Yes, sir.

Counter intuitive but makes sense the more you think of it. Brain initially wants to make a bullet going straight with sideways pressure forcing it over. Then if you move past that you want the tip downwind but that would change functional sd, which is where I was.

What’s harder to visualize is the impact of bullet shape on how far it changes into the wind. Lower drag, lower pressure differential, less shift in axis.

I have always had less wind to hold compared to my shooting partners. God, I wonder how much of the weirdness is me just doing some weird subconscious correction. I know I do it with bows. I can shoot hit 6 inches off first shot then put the pin “in the same spot” and put them all in the point of aim. Come back after a couple days and first shot is off but then I auto correct with what I swear is the same sight picture. For that reason I have to wait a long time between shots to really sight in.

 

[199p]

I had a shoot last spring that got me thinking. After mulling it over I think that ballistic coefficiency is weighted too heavily in ballistic apps and by hunters when selecting a bullet for windy work.

The example was from shooting multiple rifles and bullets in an extremely windy day. Wind force 6-7. I ended up shooting a fairly large sample size out of each rifle too. When BC’s are calculated they of course take into account the sectional density of the projectile. However, the force vector from wind has a different functional sectional density in that path of travel. The cross sectional density matters more and a larger diameter projectile will have greater cross sectional density. Basically a larger caliber even if a shorter bullet has a higher sectional density in that vector (sideways). Long bullets have greater surface area in that (sideways) vector as well. That’s not saying that those bullets can’t have less wind drift but the long sleek bullet isn’t really much sleeker in that direction of travel (from the side). Higher bc does create lower pressure wake to be acted on my wind.

My observations were that lower bc and lighter .224 bullets had less wind drift UNTIL the higher bc bullets caught up on velocity. Then the higher bc bullets had less wind drift. But it wasn’t the higher bc less wind drift for all ranges. For mine the lighter bullet had less wind drift until out past 550 yards and the higher bc bullet overtook it in wind drift very solidly at 700 yards. 69.5 hammer hbc at 2900 fps mz vs 75 eldm 2675 fps mz.

I was also shooting a 30-06 with 168 Barnes bullets with a modest bc of .47 g1 bc. Muzzle velocity was around 2780 fps. Which is around the same bc as the .224 75 eldm but with a much lower form factor. The 30-06 had about 1/5 the amount of wind drift at 500 yards as the 75 eldm in .224. More mass for the wind to move but possibly it’s more a function of cross sectional, sectional density than mass.

The .308 to .223 results weren’t surprising but the lighter .223 vs heavier higher bc was.

Basically, when determining wind drift performance when selecting a projectile I think taking into account the distance where one similar projectile vs the other reaches the same velocity is worth considering even if at the expense of bc. Kind of like how we are adopting bullets and cartridges based on impact velocity of a bullet to suitable range. Maybe add wind performance to that pre check too but consider what threshold bullet starts slower as an indicator of when that wind break even point is.

This could all be bs from a one off shooting experience. But I’ve been trying to figure out some interesting real world wind results for years. Where one bullet that SHOULD buck wind better doesn’t in back to back shooting. Who wants to finance a 1000 yard controlled wind tunnel?

Not saying you didnt see what you see but that it very differnt from what i have found,

Even at 300y ive seen a drastic difference in beating the wind going from a soft point 55grn to a 77 in my .223 then the further out the more dominating the high bc 77 is.

I dont have exact data as it was a long time ago maybe 12 years but i remember at 600 yards it was a very drastic difference in wind beating.

 

[PLhunter]

Not saying you didnt see what you see but that it very differnt from what i have found,

Even at 300y ive seen a drastic difference in beating the wind going from a soft point 55grn to a 77 in my .223 then the further out the more dominating the high bc 77 is.

I dont have exact data as it was a long time ago maybe 12 years but i remember at 600 yards it was a very drastic difference in wind beating.

The .224 bullets in question don’t have a huge gap in bc. The hammer also is going substantially faster out the muzzle while having a lower but still respectably close bc. Where a 55 grain is screaming but has a much lower bc. The 75 eldm did eventually out perform it but my observation was the distances where it did outperform were where it started to go faster.

I guess the real consideration of this post was when does faster mv overcome bc in wind deflection calculations? And I think we pay too much attention to bc for wind and not what speed we can push them. For our given actual hunting ranges.

 

[199p]

The .224 bullets in question don’t have a huge gap in bc. The hammer also is going substantially faster out the muzzle while having a lower but still respectably close bc. Where a 55 grain is screaming but has a much lower bc. The 75 eldm did eventually out perform it but my observation was the distances where it did outperform were where it started to go faster.

I guess the real consideration of this post was when does faster mv overcome bc in wind deflection calculations? And I think we pay too much attention to bc for wind and not what speed we can push them. For our given actual hunting ranges.

I havnt shot the Hammer so not meaning to say its an apples for apples

I guess its one of those things that you go with what's more comfortable for yourself.
For myself, beating the wind is basically number 1 because its so variable around me, one stream its going one way and the next it can switch 140 degrees and the windspeed is all over the show.

I think to test it, you would have to work out the average range of your shots, then see at that range what is performing the best

Shit it would definitely be boring if we all shot the same bullet

 

[Formidilosus]

I guess the real consideration of this post was when does faster mv overcome bc in wind deflection calculations? And I think we pay too much attention to bc for wind and not what speed we can push them. For our given actual hunting ranges.

It never does in the same cartridge. All else being equal, the higher BC bullet pushed to the same chamber pressure will have less wind drift in the same cartridge, than a similar designed, lighter weight lower BC bullet pushed to the same pressure. I.E.- in any given rifle, a 147gr ELD-M will always have less drift than a 130gr ELD-M from the same rifle.
There is no version of “less BC, lighter bullet, less drift” in the same cartridge/rifle.

 

[Dave Thomas]

Where does it happen? At S2H classes alone there is 50,000+/- rounds being fired in broken terrain and moderate to severe winds (10 to 60+mph) each year, from 50gr .224 to large 30cal and 338’s shot side by side.

What you wrote is not seen. All of them drift as predicted based on BC, MV, and environment.

At f class matches comparing wind holds after the match was done. I know it's just going off of what others were saying was their holds, not very scientific. I would not take it as gospel but it seems like a bigger caliber with a heavier bullet takes more wind to get blown out of the 10 ring.

 

[Dave Thomas]

Of course I haven't tested this myself as both my match rifles are 7mm and there's alot of variables not accounted for in the above example. None of us were testing this just shooting the shit after the match.