Bc vs velocity

[nhyrum]

I'm contemplating some bullet changes for my 6.5 Creedmoor.

My current load is running at 2658 fps(that was at a 20 °F powder temperature) but I have a SD of 2, and an ES of 8. Using 140 Hornady eld's.

My question is...

The heavier bullets tend to have a higher bc, but are slower, which the only advantage I see is better ability to buck wind, because the slower moving bullet should drop more, regardless of having a higher bc (although it helps negate it)

The lighter bullets, although having a worse BC, are going faster...

So......... higher bc, but slower, or lower bc but faster? Assuming I can get a similar ES and SD.

I'm looking at a bunch of bullets, ranging from Alco 120's, or 130's, lapua scenar l's 120 or 136, cutting edge mtac 133's, or Hornady eld-m 120,123, or 130's... to name a few.

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[Snyper]

I believe you're overthinking things.

If you want to try some different bullets, go for it without agonizing over minutiae.

 

[nhyrum]

I believe you're overthinking things.

If you want to try some different bullets, go for it without agonizing over minutiae.

Dang it...

Looks like I did that again... I do that a lot... like... a lot...

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[Unclenick]

Nhyrum,

Your agony may be caused by a misunderstanding of how wind deflection works. I don't have time to describe it right now, but the bottom line, worked out back in 1858 by the French, is the effect of wind is equal to the wind speed times the time difference between how quickly the muzzle velocity would get the bullet to the target in a vacuum, where there is no air drag slowing it down, and how long it actually takes to get there. That time difference is a measure of the effect drag is having on the bullet. So, get out the ballistic calculator and figure real time of flight (TOF) minus what you calculate it would be if the muzzle velocity were constant all the way to the target. Whichever bullet has the smallest difference will be least affected by wind. Elevation has to be dialed in either way.

 

[nhyrum]

Nhyrum,

Your agony may be caused by a misunderstanding of how wind deflection works. I don't have time to describe it right now, but the bottom line, worked out back in 1858 by the French, is the effect of wind is equal to the wind speed times the time difference between how quickly the muzzle velocity would get the bullet to the target in a vacuum, where there is no air drag slowing it down, and how long it actually takes to get there. That time difference is a measure of the effect drag is having on the bullet. So, get out the ballistic calculator and figure real time of flight (TOF) minus what you calculate it would be if the muzzle velocity were constant all the way to the target. Whichever bullet has the smallest difference will be least affected by wind. Elevation has to be dialed in either way.

OK, that all makes sense.

Now unclenick, my question for you is, out of the bullets I listed, are there any you have tried and like? I'd like to narrow down the list, since most are around a buck a pill.

I guess the REAL question is, would a faster, worse BC bullet "tend" to group "better" than a slower, bullet with a better bc? Or is that all nonsense

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[disseminator]

Contrary to common wisdom, the best group I have shot from my 7mm08 was with a Nosler 175g ABLR @ 2550 fps.

This "beats" a 120 Ballistic tip at 3100 since it has a much higher BC and the much faster 120g BT will face much more resistance from the air than the slower and sleeker ( higher BC ) ABLR.

After 100 yards the ABLR has more energy than the BT and gains 300 more yards of usable range. At 500 yards, the 175 has dropped 3 MOA more than the 120 BT but it's not a problem if you account for it.

Try all the loads till you find the "right" one. That is where the fun is.

 

[Snyper]

I guess the REAL question is, would a faster, worse BC bullet "tend" to group "better" than a slower, bullet with a better bc? Or is that all nonsense

The BC tells you the aerodynamic drag on the bullet. That is all.
It has nothing to do with "grouping" from one rifle to the next.

 

[mkl]

Looks like I did that again... I do that a lot... like... a lot..

Yep. You did it again.

Accuracy is primarily a function of barrel harmonics in YOUR rifle. Experimentation is the only way to find out which bullet/powder combination works best for your individual gun as far as accuracy is concerned.

Once you find the combination that works best in your rifle, then is the time to think about all the other stuff such as similar weight bullets with better ballistic coefficients, primer types, lei lines, astrological signs, phase of the moon, etc.

Experimentation once again is the only way to be sure as you play with the lessor variables. At extreme distances (600 yards plus) the BC may begin to enter into the top range of the secondary variables.

Research barrel harmonics on this forum and on google and you will find why ballistic coefficient is somewhat down the list as far as accuracy is concerned.

 

[Unclenick]

Nhyrum,

I've pretty much stuck to the 142 grain SMK in my 6.5's thus far and have not tried the others you mention.

In comparing bullets, I think you are still looking at total time of flight being quicker to reduce wind deflection, but it's the time of flight difference I outlined that actually matters. A lighter bullet typically has a shorter vacuum time of flight because you give it a greater muzzle velocity. But it also loses a higher percentage of its velocity because it is going faster, which increases drag, and because it has a lower BC, which also means increased drag. So you find the difference between actual TOF and vacuum time of flight for fast bullets can be greater than for slower bullets with a higher BC at practical velocities for both, and thus the wind deflection by the sideways component of drag is less.

For choosing a bullet based on the bullet characteristics alone, the more reasonable thing is to just plug that bullet into any ballistic calculator with a reasonable velocity and a standard comparison wind value, like 10-mph, and see how much deflection it gives you (that simplified calculator assumes a side wind). Then plug in the bullets you are interested in and fiddle with the muzzle velocity until you get a matching deflection result. Then ask yourself if you can get an accuracy load together that meets or exceeds that velocity. If you can, it is as good or better for wind than the last bullet you checked.

For example, I took the .223 Remington and looked up the 69 grain MatchKing bullet on Hodgdon’s site and averaged all the maximum velocities listed for their test gun. This came out to 2942 fps. I ran that bullet in a ballistics program and found the wind adjustment going from a 100 yard zero to 600 yard zero. I then plugged in the 52 grain Sierra MatchKing and found I would need give it a velocity of 3465 fps to get the same windage adjustment at 600 yards. But when I looked up Hodgdon’s loads for that bullet weight (actually had to use a 53 grain flat base, but close enough) the average maximum velocity was only 3275 fps. So I wasn’t likely to be able to load the 52 grain MatchKing to achieve a match, and that would be true at any range. I still might use the lighter bullet at short ranges where my hold ability wasn’t up to small difference in deflection in order to minimize recoil during rapid fire stages, but otherwise I would be better off with the 69.

Keep in mind that just because a bullet looks good on paper doesn't mean you will find a sweet spot load with it in your gun that is as good as another. At that point you have to know something about the ranges and wind conditions you will likely encounter and see whether drift or group size is more likely to cause you a problem.