Does BC rule, or bullet weight rule?

[Brad Y]

Heres an interesting one. We see BC published by bullet manufacturers but the higher BC bullets seem to be heavy 7mm and 30 cal bullets. So compare say a 0.600 G1BC 6.5mm, 7mm and 30 cal bullet roughly 140, 168 and 200gr respectively. Is any going to have more drift than others? Is there another factor other than BC that will affect results downrange?

[TOM]

Velocity perhaps.

[Brad Y]

Well yes there is another variable in the equation. But lets assume velocity is constant between calibers and so is accuracy.

[aaronraad]

If everything leaves at the same velocity, and they're all just as effcient, they will all arrive at the same time, along the same trajectory for both elevation and windage.

Read Bryan's artical The Nature of Scale and you'll see everything is proportional.

http://www.appliedballisticsllc.com/index_files/Understanding_part1.pdf

I worked up an estimate for a calibre using a similar set of calculations after I first read PO Ackley's books about 10 years ago. Glad my result was within <1% when using Bryan's formula also. Strangley I was also looking at the 6.5mm, but in 139gr configuration as a standard to pivot from.

[johnk]

Then there's that other variable - tolerances.

Most tolerances in our game are fixed irrespective of calibre, so that a tolerance has a greater potential to impact on the performance of a smaller calibre or a lighter projectile as it will constitute a greater percentage deviation from perfection. Those tolerances will not only be elements of the projectile itself, eg jacket irregularity or core weight differences, but also the elements that we introduce into the mix like consistency of powder weight, neck tension & uniformity of primer seating.

Gimme a big hole & a big case every time.

[Brad Y]

I recall Larry Bartholome talking in an article about 6.5's being blown around in a headwind more than say a 7mm. Possibly this was when they were beaten in the FCWC by the brits. I also note that a 223 and 80gr bullet seems to be brilliant out to 700yds or so but the 308's just seem to shine that bit more at longer ranges.

Would like to hear from someone that campaigns a 284 with 168gr bullets and a 6.5x284 and 140gr bullets side by side as to what performs better at the longs or in hard conditions.

[AlanF]

Would like to hear from someone that campaigns a 284 with 168gr bullets and a 6.5x284 and 140gr bullets side by side as to what performs better at the longs or in hard conditions.

Brad,

I did that for about two years, using mainly 0.301BC 142gn SMKs at about 2950 against 0.316BC 168gn VLDs at about 2840. Performance should have been about the same and I believe it was.

But I don't believe that equal BCs and equal velocities necessarily mean identical performance. Trajectory and wind deflection may be the same but stability may not. I suspect that all else being equal, a heavier projectile will be more stable in rough conditions. I've seen it again and again where the 6mms hold fantastic elevation in good conditions then suffer bad vertical when conditions cut up, whereas a 308 which may on paper have lesser performance, doesn't get affected anything like the 6mm.

Alan

[Brad Y]

Hi Alan

Yes that sounds about right and agrees with what Ive seen with the 223 vs 308. I know hot 6mm's have a case against 6.5's and 7's and 30's on paper but was wondering if there was something a bit more to it all.

Going to have a look at finding a 2950fps node in my gun when I get home.

[Norm]

I think that most of these discrepancies at long range in rough conditions can be explained.

Two bullets of different calibre but with the same “listed” BC will not shoot the same as they have a different “down range” BC.

It’s this “down range” BC that confuses things. The fact is that a bullet will have a G1 BC at a particular velocity only. Depart from this velocity and the BC can and does change. I believe that the change in BC with velocity is greater for small calibre bullets than larger calibre bullets.

The BC of smaller calibre bullets degrades quicker than larger calibre bullets. So the down range performance of these will be less in strong wind conditions than heavier/larger calibre bullets.

Using G7 BC numbers for comparisons does not help. They degrade much more than what people think. Not many people know that most G7 numbers that Berger publish have been developed over 600 yards and do not take into account the zone that is encountered further out where the velocity has dropped off. As F-Class shooters operate in this zone we are using false data to make our assumptions on bullet performance comparisons.

From my field testing with numerous calibres and bullet weights out to and beyond transonic, this BC degradation starts to become apparent once the velocity drops below 1800 fps.

[aaronraad]

Using G7 BC numbers for comparisons does not help. They degrade much more than what people think. Not many people know that most G7 numbers that Berger publish have been developed over 600 yards and do not take into account the zone that is encountered further out where the velocity has dropped off. As F-Class shooters operate in this zone we are using false data to make our assumptions on bullet performance comparisons.

From my field testing with numerous calibres and bullet weights out to and beyond transonic, this BC degradation starts to become apparent once the velocity drops below 1800 fps.

Most BC's (G1 & G7) are averaged out from 3000 fps-1800 fps (approximately Mach 3 to 1.5). These are pretty standard rifle velocities using modern smokeless powders. These velocities also correspond well with the circle segment radiused ogive (tangent/secant) nose profiles in terms of predictability and stability. However the shape is not the most efficient from Mach 1.2 and below. In fact, it's not the most efficient nose shape from Mach 5 to <Mach 0.8 region, but it is a lot easier to fabricate than some of the other profiles.

Basically the tangent/secant ogive profile is not designed to operate in the transonic and subsonic regions. Projectiles designed for less than 1800 fps are not just shaped with a nose that feeds well from a handgun magazine for a reason. There are nose profiles that operate more efficiently from below the hypersonic (>Mach 5) region, but the re-tooling costs for manufacturers is enormous and the market is very limited.

Obviously I'm working on it, but keep re-barreling and chambering for cartridges and projectiles that get you to your target at Mach 1.3 and above for the best accuracy. Alternatively you can take the 45-70 approach but she's a bit big for F-Class.

[Norm]

Aaron it just not applicable to 45-70's.
My .308 F-Standard load of 44.4gn Ar2206H and 155gn HBC hits the 1800fps mark at 570 yards.

It is only doing approx. 1185 fps at 1000 yards if you use the full BC value to calculate retained velocity. With a degraded BC it would be lower!

So given that a lot of our shooting is done below 1800 fps I think that a lot of differences can occur between two bullets with the same "believed" BC at the muzzle.

The article in the UK Target shooter mag……… http://www.targetshooter.co.uk/april-13-issue.html goes on to describe the effects of drag with reductions in stability. This is an interesting article with its methods open to discussion, but it does show that things go haywire when we operate near the limit of stability and at lower velocity.

Predicting the results of operating in this region is very difficult. It is just too simplistic to use the “listed” BC of different projectiles of different calibres and weights in a comparison. Especially with long for calibre bullets.
Field testing is the only true way to do this and the anecdotal thoughts of some of the forum members who have fired thousands of rounds over many years should be listened to. When they say that a 6mm bullet at long range in strong wind, suffers to a greater extent than a .308 or 7mm bullet of similar “listed” BC then it is a fair bet that they are right.

The question is what is the reason behind this?

[IanP]

Does BC or bullet weight rule?

It all depends Brad on if you want to be a top target shooter or a top hunter as the priority is different for these two sports.

Lets assume seeing its an F-Class forum that its about target shooting on paper targets. Then the answer to your question is really simple and could be stated as, "bullet weight is simply the result of designing a high BC bullet". Litz states in his book "Applied Ballistics For Long-Range Shooting" that generally the highest BC bullets are "heavy for calibre".

For target shooting as long as you can handle the recoil of the heavy, high BC bullets then BC rules. A higher BC projectile will always produce less wind drift than a lower BC bullet. Less wind drift with shooters of equal ability will provide higher scores.

Dont take my word for any of this as I really recommend going to the source of modern day ballistics and bullet design by buying Bryan Litz's book. None on this forum can answer such a complex question in a paragraph or two. If you want to advance as a shooter then a good basic understanding of ballistics is essential.

Ian

[aaronraad]

Aaron it just not applicable to 45-70's.
My .308 F-Standard load of 44.4gn Ar2206H and 155gn HBC hits the 1800fps mark at 570 yards.

It is only doing approx. 1185 fps at 1000 yards if you use the full BC value to calculate retained velocity. With a degraded BC it would be lower!

So given that a lot of our shooting is done below 1800 fps I think that a lot of differences can occur between two bullets with the same "believed" BC at the muzzle.

No argument with the .308W-155gr out to 1000y, but try taking it out to 3200y and starting it at only 2200 fps. A faster twist might allow the 155gr to stablise after the transonic again, but could greatly affect your supersonic accuracy.

The article in the UK Target shooter mag……… http://www.targetshooter.co.uk/april-13-issue.html goes on to describe the effects of drag with reductions in stability. This is an interesting article with its methods open to discussion, but it does show that things go haywire when we operate near the limit of stability and at lower velocity.

Predicting the results of operating in this region is very difficult. It is just too simplistic to use the “listed” BC of different projectiles of different calibres and weights in a comparison. Especially with long for calibre bullets.
Field testing is the only true way to do this and the anecdotal thoughts of some of the forum members who have fired thousands of rounds over many years should be listened to. When they say that a 6mm bullet at long range in strong wind, suffers to a greater extent than a .308 or 7mm bullet of similar “listed” BC then it is a fair bet that they are right.

The question is what is the reason behind this?

The failure of a BC alone description does not allow for other effects such as spin drift and magnus. Modified Point Mass and 6DOF programs take a lot more variables into account. They are still yet to estimate the effects of a projectile's length resulting in bending as they are assuming a ridgid body.

I think the choice of calibre and projectiles by those winning in F-Class at +600y on average speaks for itself...certainly better than most of my diatribe!