WIND DRIFT VS. B C QUESTION

[Bill Norris]

When considering match bullet performance in the wind (wind drift) does bullet weight of a larger caliber become dominant over B C and could the heavier bullet be a better performer at relative the same speed ? In other words would the heavier bullet have an advantage due to weight or B C ? (Example) - 6mm 105 gr. B C .531 @ 2900 FPS vs. 25 cal. 131 gr. B C .583 @ 2900 FPS

 

[marchx]

BC is the summary of how weight, shape, and diameter work together to resist drag and wind drift. If two bullets have the same speed, the one with the higher BC will drift less in wind, regardless of its weight. At the same MV:

• The .25 cal 131gr will drift less, because its BC is higher.
• The fact that it’s heavier doesn’t do anything extra beyond the BC.
• If both had the same BC, they’d drift the same, even if one were twice as heavy.

Bullet​	BC (G1)​	MV (fps)​	Drift (inches)​
6mm 105gr	0.531	2900	~75 in
.25 cal 131gr	0.583	2900	~67 in

 

[Bill Norris]

BC is the summary of how weight, shape, and diameter work together to resist drag and wind drift. If two bullets have the same speed, the one with the higher BC will drift less in wind, regardless of its weight. At the same MV:

• The .25 cal 131gr will drift less, because its BC is higher.
• The fact that it’s heavier doesn’t do anything extra beyond the BC.
• If both had the same BC, they’d drift the same, even if one were twice as heavy.

Bullet​	BC (G1)​	MV (fps)​	Drift (inches)​
6mm 105gr	0.531	2900	~75 in
.25 cal 131gr	0.583	2900	~67 in

Thank you Sir.

 

[davidjoe]

BC is supposed to be a true coefficient of drag, meaning that all the physical differences between two bullets are already “baked in” to the number, for drift and drop purposes.

The theory would be that if a 6mm and 30 cal had exactly the same BC, then if they were shot at identical velocities, the center of each impact would be in exactly the same place, at every distance and condition.

This is why our trajectory calculators like JBM Ballistics do not actually require that we put in the correct caliber and bullet weight to produce the drift and drop chart. It relies solely on the inputted BC for drift and drop.

 

[Webster]

When considering match bullet performance in the wind (wind drift) does bullet weight of a larger caliber become dominant over B C and could the heavier bullet be a better performer at relative the same speed ? In other words would the heavier bullet have an advantage due to weight or B C ? (Example) - 6mm 105 gr. B C .531 @ 2900 FPS vs. 25 cal. 131 gr. B C .583 @ 2900 FPS

Run the numbers thru the Berger ballistics calculator. The numbers mean nothing if the bullet isn't accurate. What are the record holders shooting?

 

[Bill Norris]

Run the numbers thru the Berger ballistics calculator. The numbers mean nothing if the bullet isn't accurate. What are the record holders shooting?

That's where I am right now deciding on a bullet. Accuracy is most important but I have other elements to consider as well.

 

[chromatica]

When considering match bullet performance in the wind (wind drift) does bullet weight of a larger caliber become dominant over B C and could the heavier bullet be a better performer at relative the same speed ? In other words would the heavier bullet have an advantage due to weight or B C ? (Example) - 6mm 105 gr. B C .531 @ 2900 FPS vs. 25 cal. 131 gr. B C .583 @ 2900 FPS

A good way to compare different calibers and loads is to calculate a figure of merit = muzzle velocity x BC. If you calculate a collection of these products, the highest performer would be the one with the highest figure of merit. This is a rough way to get close to finding your optimal caliber/load. Then you can zero in to your optimal load with a ballistic calculator.

The second step of refinement is to compare wind drift at 1,000 yards for a 10 mph cross wind with your ballistic calculator. In F-class, 60 inches is a good number; 90 inches is not.

 

[Jason D Hostetter]

No one ever seems to account for form factor and no program I'm aware of uses a G8 reference. So, we're stuck with using a G7 bc as the best case, G1 worst case. In the field, I've noticed that regardless of BCs being very close or identical, the smaller diameter bullet has less surface area for the wind to push against, and I've noticed I have to hold or dial less wind with 6mms and particularly 224s, than what the calculation calls for. I believe that we'd have to more complex ballistic solvers, using sectional density, true diameter and the length of said diameter, form factor, and a G7 or G8 BC, to obtain the absolute best solution. Even then, human error in data entry or a bad wind call, still puts you back outside of where you want to be. Example of the previous, I typically shoot field steel matches with my custom 223, which is a 30" 7 twist .090" freebore, I shoot 80grn 80ELDMs at 3206 avg with PP2000MR and a corrected G7 of .239, which has worked for me, all the way to a mile, and does phenomenal in our typically setting of 1130yrds and in. However, I'm typically using around 60% of the wind call from the solution. If it calls for 6moa, my hold is typically 4-4.25moa. Bryan Litz and a couple others address these in more detail, but there is indeed more to it, than just velocity over G7. Mostly it's over looked because a lot of people are shooting moa or 2 moa sized targets and being a few percent off in your solution, is still a hit, and a minor correction on the fly, can center it up. Unfortunately most don't take notes on what they had to do, to center it up. Sorry for the long winded answer.

 

[xr650rRider]

JBM has a drag function conversion utility. I believe the G8 drag model is also available under the trajectory calculators. Another thing to think about wind drift is time it's in the air for wind to work on it. Faster it is, less drift. A lot of the time, what we quickly jump on as an error in the calculation for drift, is simply that your scope is canted in the direction to cancel it out.

https://www.jbmballistics.com/cgi-bin/jbmgf-5.1.cgi

 

[J_idaho]

I’ve pondered and searched this allot. Because real world wind calls never coincide with kestrel. But who can make an absolute wind call when any terrain is involved. From everything I’ve read and watched the ending answer is probably what angle the tip of bullet is at in flight along with how perfectly smooth it’s spinning because the tip is what steers the bullet wind hitting side of bullet has little to no effect on flight. So in the end what is it doing on target will be the only way to see.

 

[gmr4]

BC consistency is important. Im pointing with the MTAP now and the speed at target is slower than un pointed but the pointed, trimmed and pointed, group much better… that result is from shooting 500 of each pointed and non pointed. So I’d choose a bullet known for consistency rather than just higher BC.

 

[Jason D Hostetter]

JBM has a drag function conversion utility. I believe the G8 drag model is also available under the trajectory calculators. Another thing to think about wind drift is time it's in the air for wind to work on it. Faster it is, less drift. A lot of the time, what we quickly jump on as an error in the calculation for drift, is simply that your scope is canted in the direction to cancel it out.

https://www.jbmballistics.com/cgi-bin/jbmgf-5.1.cgi

People should indeed use a level and a plumb line to remove any cant in the optic, and should align directly behind the scope with a full picture, no shadowing. I use a Sendit level in 2 spots on the base and another one the chassis, vertically mounted with an mlok rail, to ensure the rifle is indeed level, and I have a plumb bob hanging at 50ft, to ensure the reticle tracks with the level. I also zero a quarter minute or tenth of a mil left of dead center, to avoid any compensation for spin drift, since I only use right hand twist barrels.

 

[Bill Norris]

JBM has a drag function conversion utility. I believe the G8 drag model is also available under the trajectory calculators. Another thing to think about wind drift is time it's in the air for wind to work on it. Faster it is, less drift. A lot of the time, what we quickly jump on as an error in the calculation for drift, is simply that your scope is canted in the direction to cancel it out.

https://www.jbmballistics.com/cgi-bin/jbmgf-5.1.cgi

From my limited experience with shooting in the wind and trying to learn to read it I see it is very telling if the scope is installed with cant. I feel the tall target test @ 100 yds. is mandatory to remove the cant and make it a part of every scope installed.

 

[xr650rRider]

Even the way the barrel is chambered and clocked can effect windage and elevation. If you have a 16 TPI thread and someone allowed .002" of crush and when tightening the action, it had a really square shoulder and only moved .001", then you might be 6 or 7 degrees out of perfect vertical. Now does it add or subtract? Depends on the direction.

AssUmes you dialed in the throat and let the muzzle wander.

 

[SeabeeKen]

From my limited experience with shooting in the wind and trying to learn to read it I see it is very telling if the scope is installed with cant. I feel the tall target test @ 100 yds. is mandatory to remove the cant and make it a part of every scope installed.

Brings a recent experience to mind Bill. Bought a sightron s3. Watched to crosshairs turn with each shot, eventually getting to 45*. There were never properly aligned. Sent it back. Next scope same thing. Sent it back and got another that was mostly aligned. Was told by cs that they went thru 5 scopes to find that one. Then a big debate about crosshair in alignment with turret. No more sightrons for me. Then at the next shoot, the dealer was shooting a nightforce, lol.

Noticed ballistic apps have a truing feature.

 

[ELR LVR]

That's where I am right now deciding on a bullet. Accuracy is most important but I have other elements to consider as well.

That is sometimes the dilemma
the choice between a more accurate bullet vs
(A Bullet with a much better BC but may be slightly less accurate)
IE: Lapua 105 vs 115 Dtac
It kinda depends on the distance being shot to where the BC gives the advantage you are looking for.
The higher BC bullet may be less accurate (if only very slightly) at short distances by comparison of group sizes.
but at greater distance is actually MORE accurate due to its superior BC

 

[Steve Donlon]

No one ever seems to account for form factor and no program I'm aware of uses a G8 reference. So, we're stuck with using a G7 bc as the best case, G1 worst case. In the field, I've noticed that regardless of BCs being very close or identical, the smaller diameter bullet has less surface area for the wind to push against, and I've noticed I have to hold or dial less wind with 6mms and particularly 224s, than what the calculation calls for. I believe that we'd have to more complex ballistic solvers, using sectional density, true diameter and the length of said diameter, form factor, and a G7 or G8 BC, to obtain the absolute best solution. Even then, human error in data entry or a bad wind call, still puts you back outside of where you want to be. Example of the previous, I typically shoot field steel matches with my custom 223, which is a 30" 7 twist .090" freebore, I shoot 80grn 80ELDMs at 3206 avg with PP2000MR and a corrected G7 of .239, which has worked for me, all the way to a mile, and does phenomenal in our typically setting of 1130yrds and in. However, I'm typically using around 60% of the wind call from the solution. If it calls for 6moa, my hold is typically 4-4.25moa. Bryan Litz and a couple others address these in more detail, but there is indeed more to it, than just velocity over G7. Mostly it's over looked because a lot of people are shooting moa or 2 moa sized targets and being a few percent off in your solution, is still a hit, and a minor correction on the fly, can center it up. Unfortunately most don't take notes on what they had to do, to center it up. Sorry for the long winded answer.

A lighter bullet has less mass for the wind to push against but also less mass to push back against the wind. I thought a lighter bullet slows down faster than a heavy bullet. At some point, can it go the other way because of the air resistance and angle of the wind?

 

[CharlieNC]

Bill a ballistic calculator is your friend and is reasonably accurate, so run various options to narrow down candidates to test. I have found several that look best on paper just didn't shoot.

 

[10XSHOOTER]

Having been a long range shooter in the Navy.! I have always said on a windy day. I would take my .300 win mag with a 190 Sierra Mk over my 6.5 X .248 with 142 Sierra Mk. Both at 2900 Fps

 

[davidjoe]

A lighter bullet has less mass for the wind to push against but also less mass to push back against the wind. I thought a lighter bullet slows down faster than a heavy bullet. At some point, can it go the other way because of the air resistance and angle of the wind?

If two bullets were dimensionally identical, but one was heavier than the other, then they would indeed present identical profiles to the wind, but the wind would have less effect on the path of the heavier bullet.

This would be the case of one turned solid from copper versus the other from tungsten. If the shapes of two are identical, and it is only “shape” or “form” that the wind contacts and acts upon, then the wind does see them similarly, at least initially. But the two don’t have the same BC because BC incorporates sectional density.

The denser bullet has more inertia in its direction of travel for the wind to try to overcome. If the two weren’t even moving at all, and wind blew across them, the lighter bullet has less resting inertia and would be the first to move.

If two bullets have the same BC, the lighter bullet really should be expected to “carry” as far and to the same place as the heavier one, assuming same MV. This means even their drop at all chosen distances is the same (come ups are also identical). Further, they will fly parallel arcs and be side by side the entire path, until they hit the ground, if fired together side by side. That’s the harsh but pure working definition we have of BC. The difference is the amount of energy each arrives with.