How much difference does the BC for the 260 really make in terms of trajectory or down-range performance? I always hear how good it is but I would appreciate it if someone could help put it into perspective for me. Please give me comparisons to other calibers like the 270, 280, 308 if possible.

Nebraska,

Here is the JBM on line ballistic program

http://internet.cybermesa.com/~jbm/ballistics/calculations.html

Have a play with it and you will have the answers.

Mike

I assume you are asking for comparisons of like-weight bullets between calibers?? 6.5mm (.264")bullets usually have a pretty high B.C., but they are not greater than the B.C. of bullets of other calibers having the same sectional density and form factor!! You have to be a little more definitive when asking a question like the one you posed. For example, while a 140 grain 6.5mm bullet might have the same B.C. as a 180 grain .308" bullet, the 180 grainm .308" can be driven as fast as the 6.5mm, using similar pressures, due to its' larger base area.

Sorry, couldn't get the chart to print out properly.

[ 06-21-2002, 21:40: Message edited by: Bruce Gordon ]

I believe there is confusion about b.c. (ballistic coefficient) and s.d. (sectional density).

6.5 mm bullets have very high sectional densities in 129 gr and up bellet weights. B.C. is based upon mostly the shape of a projectile, not the diameter.

High b.c. bullets buck the wind better and shoot flatter then low b.c. bullets, all else being equal. High s.d. bullets will penetrate better, once the target is struck. Some 140 grain bullets in 6.5 mm diameter do have high b.c.s, which gives you the best of both worlds.

Ballistic coefficient is a function of BOTH shape (FORM FACTOR) AND sectional density. Each of these factors IS EQUALLY IMPORTANT in determining the ballistic coefficient of a projectile!! The best shaped bullet in the world cannot overcome air resistance if it has no weight!!

First, to address the original question a bit more in depth.

Not all 6.5 mm bullets (even the heavy ones) have high B.C.s (the original statement in the question). A Hornady 160 grain, 6.5 mm, round nose bullet has a high S.D. of .328, but a fairly low B.C. of .283. The Barnes 100 grain solid spitzer in 6.5 mm has a low S.D. of .205, but a good B.C. of .395. Therefore, to assume that a high S.D. bullet has a high B.C. is incorrect, as well as is stating that 6.5 mm bullets have a high B.C. in general.

eldeguello posts, "Ballistic coefficient is a function of BOTH shape (FORM FACTOR) AND sectional density. Each of these factors IS EQUALLY IMPORTANT in determining the ballistic coefficient of a projectile!!" with such vehemence that I simply must take exception to it.

There are several formulas for calculating B.C., but none are as accurate as actually shooting hundreds of chronographed rounds at various distances to get an actual B.C. vs. a computed B.C. The simplest equation that I have found is: C = SD/f, where C=B.C., SD=Sectional Density, and f=form factor. Obviously, in this equation, SD and f are equally important in determining B.C.

Now then, the problem with assuming that a high SD automatically means a high BC is that form factor is a very complex thing, and small changes to the bullet's shape can make a big difference in the form factor number. Therefore, the divisor in the above equation could change dramatically with a small change in shape, and thusly influence the outcome to the same degree.

The terms "form factor" and "shape" are not interchangeable. Form factor is merely the attempt to mathemnatically compare the shape of one bullet to the shape of another bullet with an assigned form factor.

Small changes to the SD would result in correspondingly small changes to the above result. SD is simply weight/squared diameter.

That is the basis upon which I stnd by my statement that shape has a greater influence on B.C than does S.D. SD and FF have the same influence, but FF can be drastically changed by a small change in shape.

I hope this clears things up a bit. Notice my post did this without any CAPITALIZED statements, or utiliztion of multiple punctuation!!!

I find it rather telling about a person when they feel the need to graphically "yell" at someone whom they've never personally met when all that person has done is type a few lines that they disagree with...

[ 06-23-2002, 23:58: Message edited by: 6.5 Guy ]

6.5 Guy, I certainly agree with you that a high sectional density does not convey a high ballistic coefficient, because, as you noted, the form factor is the other component!! AND, I also agree that to determine B.C. absolutely, one must do firing tests with a particular projectile. As a matter of fact, the B.C. of a given projectile can vary somewhat just because of different muzzle velocities. I also agree that when using SD and form factor to calculate B.C., the result is merely an approximation, and sometimes not a very good one. I believe it is for this reason Sierra quit publishing the B.C. of their bullets-more and more people were getting chronographs, and often actual test results were not commensurate with the B.C.'s Sierra had published!! The point I was trying to make was that when Nebraska was asking his question, he did not ask it specifically enopugh to give him a meaningful response. "

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Compared to what, Nebraska??

OK Larry, no sweat. I was jsut trying to help clarify what I thought Nebaraska's question to be, and the answer I thought would be most helpful to him.

Thanks guys. The gist of my question was really to spark some dialogue (like we have going) on the bullet design and ballistics of the 6.5mm bullets in comparison to other bullets in the 24-30 caliber range.

It's exchanges like these that really add depth to my (little)understanding of ballistics, bullet performance and the like.

As I'm so inexperienced, I often use incorrect terms when asking or responding to questions but I'm getting better by listening to you guys.

What makes me curious about the 6.5 is that (to me) the bullet is often touted for it's superior performance and I wanted to know the actual reasons why from the guys who know. Is it because it's realatively heavy in relation to it's diamater, resulting in a bullet that doesn't have much wind resistance and can also penetrate deeply for the same reason? If that's so, why don't they just offer longer, heavier bullets for all calibers?

I find many of the exchanges/debates on this board absolutely fascinating. I've been very entertained and enlightened by the folks who visit this board. In addition to having an in depth understanding of hunting, shooting, reloading, etc. the majority of folks luckily have a very good way of relating their knowledge in a way I can understand and appreciate.

Thanks and keep it up!

Nebraska, actually the 6.5mm is a great bore size. If you compare like-shaped spitzer bullets of similar weights, for example, a 150-grain in the .30 caliber vs a 140 grain in the 6.5mm, and start them at similar ve;locities, the 6.5mm will retain velocity and energy better than the .30. In addition, heavy-for-caliber (140 to 160 grain) 6.5 mm bullets generally provide considerable penetration at medium to low velocities. A friend of mine once shot a large mule-deer buck with a Sewdish Mauser carbine using a handload with a Hornady 160-gran round-nose bullet. When he went over to the fallen buck, which was about 80 yards away, he found a dead doe about ten yards beyond the buck which had been killed by the same bullet! The slug went completely through both deer and off across the canyon!

Nebraska,

As you might have surmised from my user name, I am a fan of the 6.5 caliber. I have rifles in 6.5x55, .260 Rem, .264 Win mag, and 6.5-284. I like them all, and have no real "reason" (that my wife would understand anyway) to own all of them other than to fiddle around.

In any event, yes, the 6.5 mm cartridges, when utilizing heavier for caliber bullets, do have great penetration qualities that allow them to compete against larger caliber when it comes to effective killing power.

You asked about using heavier-for-caliber bullets in larger calibers for the same effect. Sure, you can do it, and many do. Traditionalists oftentimes load up their .30-06 Springfields and .30-40 krags with 220 grain round nose bullets. They work great too. There are a couple of problems using the heavy bullets in .30 calibers that are not really as much of a consideration with the 6.5 (or 7mm, for that matter) calibers.

The first one is recoil. A 220 grain slug propelled at a max velocity in an '06 is going to belt you pretty hard. Much harder than a 140-160 grain pill in a 6.5, or 160-175 grain bullet in a 7mm. That's the first part.

The second one is trajectory. The .308 or '06 case is really at it's limits with 200-220 grain projectiles. There simply isn't the powder capacity to get those really big slugs moving at the velocities most people want nowadays (2600+ fps) Hence the advent of the .308 Norma, .300 Win Mag, etc. cartridges. Not so with larger 6.5mm and 7mm bullets. The Swede, 6.5-'06, 7x57, and .280 Rem have plenty of oomph to send the larger bullets out their chutes with good velocity(2600 to 2800 fps).

I REALLY like my .264 Win Mag with 140 grain Partitions, and my 7mm Rem Mag with 160 grain X bullets. The s.d.s for these bullets are .287 and .283 respectively. These are quite high (a 200 grain .308" bullet has a s.d. of .301), and they give me the velocity (3150 and 3090 fps) that I want for the long shots I take in the open areas I hunt (much like where you hunt, I imagine). I don't have to get pummelled by a .30 caliber magnum to get the flat trajectory I need, and these bullets are plenty big and strong enough to penetrate deep and kill quickly on animals like deer, pronghorn, sheep, elk, and moose.

Long post, but it sounds like you were asking for it!

I reread Larry's post, and thought I'd throw in a 6.5 mm bullet story as well. This took place with a 140 grain X bullet with a muzzle velocity of 3,200 fps in my .264 Win mag.

A few years ago, I took a long shot (for me anyway) at a whitetail. I hit him, but had misjudged the crosswind, hitting him a bit further back than I would have liked to. I couldn't tell if he was hit far back in the lungs, or gutshot. He took off like a dart, so I shot twice on follow up, trying to bring him down. He was heading straight away at full speed, and my third shot took him down to the ground.

I paced the distance off. My first shot was at 330 yards, the second was about 340, and the final one (which knocked him arse over teakettle) was at 370. The second shot missed, but the third one entered him about five inches above and in front of the base of his tail. It broke his back, moved forward and down through his lungs, and angled to the left to break his shoulder. The bullet was embedded in his shoulder, after penetrating some 30" or so, and breaking two big, heavy bones.

It turned out that my first shot wasn't as bad as I had feared, and his lungs were both hit. I don't know how far he'd have gone before succumbing, and I'm glad I didn't have to find out, as it was getting pretty late.

I think the .264 Win Mag and 7mm Rem Mag (with medium-heavy bullets) are the best calibers for use on the Great Plains for deer and pronghorns.

I see one thing that you made no mention about, that I see as very important to the overall equation. Many 6.5mm guns have a twist rate of 1:8" which is capable of stabilizing the heavy for caliber bullets, whereas many 7mm & 30cal guns have a twist rate around 1:10" or even slower (yes I know that 7mm generally has 1:9.5" but I'm considering that close to 10") which is not capable of stabilizing long spitzer heavy for caliber. This makes it a practical matter of whether your rifle will actually shoot those high BC bullets which may or may not be available for your caliber of choice.

Bret

Twist rate is definitely something to take into consideration when selecting a bullet for a given caliber. I've developed 175 grain partition loads for two different 7mm Rem Mags (Ruger and Winchester) and had no problems with bullet stabilization.

Nebraska,
A friend of mine has a 260 Rem. and shoots only 142 gr Sierras. His 600 yard elevation adjustment is 2.5 to 3.5 MOA less than the 308 Win. with 175 Sierras. The rifle noticeablely recoils less and pretty much outshoots the 308 at all yardages.
Todd