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Do bullets with same BC & velocity but different weight/caliber drift the same??

If this topic has been discussed I apologize for this post.

I hear this a lot lately that if two bullets, different caliber/weight, but with same BC are fired at same velocity the bullet that is heavier(bigger caliber) will drift less.
For example Berger 185 fired from 308 at 2750 fps will get less affected by wind compared to Berger 90 VLD out of 223 at 2750 fps, even though G7 BC is almost the same. Ballistic solution suggests they should both drift the same but some 'experts' disagree.
So my question is, isn't BC supposed to include all that? The way I thought was that BC is the final number that tells what bullet will do in flight. Is there more to it? Extra weight/momentum has something to do with flight characteristics also?
Brian I really appreciate your help that we all get through this forum.
Thanks
Av
 
BC is a number that boils down the drag behavior of a bullet (which varies greatly with velocity) to one number to make comparisons (and calculations) easy. Among other factors, it already accounts for the mass of the bullet. <- read this sentence again, for emphasis.

If you believe that wind deflection is caused by drag, which is the overwhelmingly conventional wisdom, then yes, two bullets with the same BC and velocity will deflect the same in the wind.

That said, BC is generally an imperfect approximation of actual drag, so there can be VERY SMALL differences in actual performance between bullets that have "the same" BC. Further, measuring BC is not as exact as we'd like it to be, so two bullets with "the same" BC may have different real BC's simply due to measurement uncertainty.
 
I'm very interested to hear the responses from your question. I'm guessing the answer is NO. If it was true, why wouldn't everyone be shooting 223 in FTR instead of 308? But, most people shoot 308 and swear that past 600 yds is where the heavier bullet shines.

Great question. Looking forward to the answers.
 
Calgarycanada,

keep in mind that ballistic coefficient was invented by Francesco Siacci about 1880. It is a very dated concept from the time when computing was a very time-consuming process.
BC does not cater for drift at all.
On the other hand, drift is totally submerged in the effects of crosswind. In my view, considerations about drift are interesting to the ballistician, but a waste of time for practical shooting.
Bryan Litz has published in his book a formula for estimating drift, which includes the effects of bullet stability. It is in excellent agreement with the U.S. Army firing table data for the .30 M2 ball cartridge, which were obtained by experiment.
 
r bose said:
I'm very interested to hear the responses from your question. I'm guessing the answer is NO. If it was true, why wouldn't everyone be shooting 223 in FTR instead of 308? But, most people shoot 308 and swear that past 600 yds is where the heavier bullet shines.

Great question. Looking forward to the answers.
Except that the velocity from the two bullets would not be the same over the full duration of the bullet flight to the target
 
JabaliHunter said:
r bose said:
I'm very interested to hear the responses from your question. I'm guessing the answer is NO. If it was true, why wouldn't everyone be shooting 223 in FTR instead of 308? But, most people shoot 308 and swear that past 600 yds is where the heavier bullet shines.

Great question. Looking forward to the answers.
Except that the velocity from the two bullets would not be the same over the full duration of the bullet flight to the target
It's quite possible that ballistic solutions are incorrect but shooter app shows same remaining velocity at 1000 meters for both bullets in my example(original post)
Mr Litz have tested both bullets that I'm using as example so BC should be as accurate as it gets. I'm really interested to see what Brian has to say about it.
 
calgarycanada, I believe you hold the correct perspectives on this. BC does take care of it, where BC is truly the same. This of course with all else equal(same MV, etc).
Drag slows the bullet, gravity sets the drop rate. Same drag, same gravity, same drop.
 
mikecr said:
calgarycanada, I believe you hold the correct perspectives on this. BC does take care of it, where BC is truly the same. This of course with all else equal(same MV, etc).
Drag slows the bullet, gravity sets the drop rate. Same drag, same gravity, same drop.

Are you telling me so called "Experts" could be wrong?? The ones who believe heavier bigger caliber with same BC (185s in 308)is better in wind then smaller caliber(90 VLD in 223)?
 
It also must be remembered that the BC changes with stability. If we shoot a 185 Juggernaut at a Sg of 1.7 and a 90 vld at 1.2 the 90's BC will be negatively affected by the oscillations that the bullet does enroute to the target.

Bryan notes this effect in his books, and has also commented on stability effects on BC here.

So, the shooters might have observed something (more wind drift for the 90's) and simply credited it to the mass of the bullet. We humans have a tendency to jump to conclusions. So the experts might be right that for a published BC, the wind drift was not equal. The difference might have been more attributable to the effective BC.

If I were building a 223 F/TR rifle, I'd contact Bryan and ascertain what barrel twist was used for his BC determination.

All of that information and $5 will get you a latte....
 
Where the 'form factor' is the same but bullet weight varies, the heavier bullet is more ballistically efficient when fired at equivalent BCs. This seems strange as the form factor is the key measure of airdrag efficiency, measured in relation to the 'reference projectile' which always has a value of 1.000.

Increase the weight and the BC rises, but velocity falls, so 'commonsense' says that if two bullets are equally efficient they will perform the same. However, not so - the heavier model 'wins'. As an example, Bryan Litz has given two 30 cal bullets i7 (form factor) values so close as to be treated as identical within experimental methodology accuracy used to obtain them - the 175gn Berger LRBT (0.999) and the 210gn Sierra MK (1.000).

Give them comparable MVs based on ME values. Let's say the 175 can be fired safely @ 3,000 fps = 3,498 ft/lb ME. Translate to the MV that gives that value for a 210 and you get 2,739 fps MV.

Compare the two combinations in a ballistics program
The 175gn Berger has 0.264 average G7 BC. 1,000 yd at 3,000 fps MV = 1,447 fps (1.28 MACH) retained vel / 77.7” drift in a 10 mph crosswind.
The 210gn Sierra has 0.316 average G7 BC. 1,000 yd at 2,739 fps MV = 1,473 fps (1.31 MACH) retained vel / 70.3” drift in 10 mph crosswind.

However, two bullets with identical BCs, and assuming they are equally well designed for the distance application, so one doesn't produce tail-end turbulence for instance, should drift the same amount in a given wind after discounting spin-drift which may vary if we're into different calibres and rates of bullet rotation needed to stabilise them.

In my days of shooting a 223 with 90gn VLDs and 308 with the 185gn Juggernaut with near identical BCs but the 223 at a higher velocity (2,910 fps MV v 2,827 fps) I had little doubt that the former moved less in the wind. However, no matter how careful one is in loading 223 it suffers the curse of the occasional shot producing unexplained 1,000 yard elevation variations whilst the 308 / 185 combination held amongst the best elevations I've ever managed to obtain.

I do also believe that straight (crude!) ballistics results whilst very useful are only part of the puzzle. 'Tuneability' and 'shootability' are important, and there are intangibles which see a bullet perform better or worse than its ballistic peers, or maybe suit more conditions in more matches. This isn't just a matter of BC and MV, but may include factors such as rifling twist / bullet rotation rates.
 
I don't think there is a difference in wind drift between the two bullets you chose at the same muzzle velocity assuming that the stability factor is the same or similar as well. I agree with Laurie that most F-TR shooters choose the 308 Win to get better tuneability. Plus, with a 308 Win, you are in greater control of the ammo due to the larger case volume. In 223, the velocity spreads are more dependent on primer consistency than with 308. Also, slight differences in case volume are less noticeable in 308.

If you are going to shoot the 90 VLD in 223, you have to be super OCD about case prep, charge weighing, and bullet sorting because the small differences make a bigger impact on target. I love the 223, and have shot it in competition, but I won't take it to any serious matches. It's just to hard to control all of the reloading variables. YMMV
 
BCoates said:
I don't think there is a difference in wind drift between the two bullets you chose at the same muzzle velocity assuming that the stability factor is the same or similar as well. I agree with Laurie that most F-TR shooters choose the 308 Win to get better tuneability. Plus, with a 308 Win, you are in greater control of the ammo due to the larger case volume. In 223, the velocity spreads are more dependent on primer consistency than with 308. Also, slight differences in case volume are less noticeable in 308.

If you are going to shoot the 90 VLD in 223, you have to be super OCD about case prep, charge weighing, and bullet sorting because the small differences make a bigger impact on target. I love the 223, and have shot it in competition, but I won't take it to any serious matches. It's just to hard to control all of the reloading variables. YMMV
Good points about reloading consistencies. I'm Just trying to bust the myth that lighter bullets get blown around by wind more even if they have same or higher velocity and same or better BC then heavier and bigger caliber bullets.
 
Bullets having the same BC and MV will have the same trajectories; drop, drift, retained velocity, tof, etc. The only difference will be energy.

Of course it's possible that one bullet is under-stabilized and not realizing it's full BC potential.

Consider a comparison like the 90 grain .224 cal VLD vs. a 185 Juggernaut (G7 BC's of .281 vs .283). The average BC's are very close for two very different bullets. However it's possible that the BC's change differently with flight speed. In this case, you would have to know the velocity dependent BC's of each bullet. Considering that the BC's (.281 and .283) are G7 BC's (don't change much with speed) and are averaged from 3000 to 1500 fps, it's not likely that you'll see much if any difference in the trajectories (including wind deflection) at long range.

So why do you see so many .308's on the line and less .223's?

My answer; the .308's can easily shoot 155.5's and 185's into small groups reliably with no pressure problems all day long. To match this wind performance with the 223 at 1000 yards, you have to run the hottest loads with the longest heaviest bullets, running the fastest twist barrels and even then, I don't know anyone who's successfully been able to campaign a 223 for a full season at 1000 yards with reliable accuracy. 600 yards, of course. You can use less radical stuff at 600 like 80 grain bullets and 1:8" twist barrels, moderate loads, etc. But to get the performance at 1000, you have to really push the 223 on the knife edge and it tends not to work well in the long run.

Yes, on paper the wind deflection is equal for the two. But in reality, the trouble you have getting the .223 bullet to the speed it needs to match performance with a .308 is difficult. Furthermore, I think in this specific comparison there may be something else at play. As the .308 bullet (155.5 to 185 grain) slows down to low supersonic speed (below 1500 fps) as it approaches 1000 yards, it's stability remains in good shape. It's not that long of a bullet for .30 cal. However, the 90 grain .223 bullet is having a difficult time with stability at this speed, and may be flying with a great deal more pitching/yawing in the last couple hundred yards. In other words, due to the reduced stability for the long 90 grain VLD, it may not be realizing it's full potential BC, and therefore might be deflected more in the wind.

Note the paragraph above is an educated guess, as I've not tested the 90 grain VLD specifically at these speeds. Just offering a possible explanation for why something may seem one way on paper, but turn out different in reality.

Things like that can be confusing.

Take care,
-Bryan
 
So why do you see so many .308's on the line and less .223's?

My answer; the .308's can easily shoot 155.5's and 185's into small groups reliably with no pressure problems all day long. To match this wind performance with the 223 at 1000 yards, you have to run the hottest loads with the longest heaviest bullets, running the fastest twist barrels and even then, I don't know anyone who's successfully been able to campaign a 223 for a full season at 1000 yards with reliable accuracy. [Bryan Litz]

Three answers I guess, Bryan. One is that many shooters and l-r FTR or fullbore shooters simply cannot or won't accept that the 223 can be competitive at 1,000 yards. When I started shooting with the 80gn Sierra MK at 800 and above, people literally didn't believe I was doing it until they saw it with their own eyes. Although that situation has eased now, people still have trouble accepting the 'mouse gun' can be competitive at these distances.

I had a very consistent period with my first 223 built for 90s with 7th overall in the GB FTR league in 2011, a score that briefly was a joint 15-round 1,000 yard GB FTR record, second highest team shooter aggregate in the 2011 Scotland v USA team matches shot over 900, 1,000, and 1,100 yards at Blair Atholl in June 2011, and in the preceding individual matches an outright 1,000 yard win against competition which included six members of the then US FTR team. That November I got my highest ever FTR placing in the European F-Class Championships, 14th or 16th, can't remember exactly now and shot a coached aggregate score in the Sunday team matches at 900, and 1,000 that was almost exactly on the mid-point of the GB #2 team scores and helped them take second place to GB #1. All GB league round matches were shot over a minimum 800 yards, but in practice nearly all were 1,000.

Third, getting that sort of consistency is really hard work though and absolutely everything in the rifle, barrel, chamber and ammo has to be just so. There isn't the latitude that 308 gives. Also, we don't have the temperature shifts in the British Isles that you see in your matches, and that is undoubtedly an issue as temperature effects are going to create bigger changes on the little cartridge's performance.

308 bullets have come on a way or two as well since 2011, whilst 224s haven't. People over here are shooting their 155s 100 fps faster than they were four years ago too, while my latter 223 loads saw a 60 fps fall over what I was using back then for various reasons I won't go into. However, we have a new range of Nitrochemie 'Reload Swiss' powders here now including a 'high-energy' grade that looks ideal for 223 with 80/90s and which incorporates the self-same EI technology that allows nitrochemie manufactured Alliant Re17 to get 50-150 fps more than the 4350s in its burning rate niche. So, I'm going to get my (two) 223s out of the cabinet and see if I can get the 90gn VLD up to the right side of 2,900 fps where it seems to need to be to get the consistency at long-range.
 
Here is an "on paper" chart I put together using the applied ballistics site. It's good for finding lag time/wind deflection equivalents (points on the same horizontal line).
It does NOT assume any stability issues due to spin or passing below the supersonic range, pressure problems from pushing MVs to the limit, etc.

Notice the 22 and 6mm curves have about the same BC, so they are right on top of each other. The 308 curve is just below in orange.

The 308s seem to have some inherent BC limitations due to their generally higher G7 form factors.
As you can see, even the lighter 140 gr. 6.5mm and the 180 gr. 7mm beat out the 185 gr. 308... assuming constant chamber energy.
 

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I say, forget the 308 and start thinking 338!!!! [Joe Grad]

Yes, if you can shoot it consistently over a match. We used to have a couple of guys in my club who shared a Sako TRG in 338 LM back in the very early days of F-Class when F was F was F and nobody had invented FTR. The bigger 6.5s had only just become the cartridges to use in the discipline.

It was a l-r match, 900 or 1,000 - can't remember which - with a typical gusting 20 mph plus Diggle westerly wind blowing up the range from 4 1/2 to 6 o'clock. I was shooting a 223 with 80gn SMKS at a not particularly high MV, certainly not enough to keep them supersonic at the target, and peaked out at 12-MOA right wind. My mate shooting a 308 with 155 Scenars that we all shot then was running at ~10-MOA max, one of the club's top scorers recently moved to 6.5X55 AI was using around 6 1/2 (Impressive! I thought) and the guy with the 338 said he never got above 3 1/2. (Wow!!) Even taking into account people's inaccurate wind zeroes, that seemed pretty instructive.

But the guys with the 338 never got anywhere with it, even on the old NRA target with a 2-MOA Bull we shot on then. They just could not be shot accurately enough. They or somebody else in the club also used a 338 in 1,000 yard BR and Vince Bottomley (well known to people on Accurate Shooter) once told me it shot 10, maybe 12 inch groups in fair weather or foul. So they were right down the bottom of the results list on any of Diggle's occasional gentle days, and did better but never near winning anything in more normal gusty conditions!
 
One thing I have not seen in this thread yet, assuming the same BC and starting velocity, the lighter bullet will shed it's velocity faster, thus lowering it effective BC at this reduced velocity, plus increasing it's time of flight. The heavier bullet will have a higher remaining velocity and a higher BC at the 1000 yd mark with less TOF and thus less wind drift, IMO.
 
Thank you very much Brian and Laurie.
Just one last question (I promise, maybe ;D) so does 90 grain VLD have design issues or is it just that 223 case is not sufficient for it? Let's say if 90 VLD is launched out of BR or variants to same velocity as 6mms don't you think it's better combination (on paper anyway) upto 600mts? It does have higher BC. Is there something more to BR type(short stubby) cartridge and 6mm bullet combo?
 
To kodiaks comment... Assuming a modern boat-tail bullet is used that matches the g7 standard, the g7 bc will stay about constant for varying supersonic velocities.

A lighter bullet and a heaver bullet will fly the same trajectory if g7bc and mv are the same. This means they will slow down the same and deflect the same due to wind, regardless of weight or caliper. BC (which incorporates weight, caliper, and form factor) and mv are all the wind sees.
 

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