Ok from what I understand SD (sectional density) is the density of a bullet compared to it's weight and length ! Is that correct ?
In other words you can have a 300 gr 375 bullet 18mm long and another 16 mm long also 300gr and the one that is 16mm long as more SD than the other.

Am I talking bull or am I on the right path ??? Then if it is so, does SD make a difference when hunting and why ? And what is the best SD to use and how do you measure SD of a bullet ???

Lots of questions but this has been bugging me for a while now.

SD is the weight divided by the cross sectional area. The higher SD the better the bullet holds it's velocity and the more it will penetrate. The larger the animal and the more dangerous it is ,the higher the SD that should be used.

Mete:

I think you have it correct, but stating that "momentum" is retained might be a better way to state it.... that, is, of course, the tendency for "objects in motion (to) tend to stay in motion."

You can have lots of sectional density, but if the bullet sheds weight during penetration, then, obviously, the s.d (and penetration) will decrease accordingly. That's where the new bonded and monometal "softs" shine. Little weight is lost with them, even though expantion is good, one gets greater penetration than a bullet that shucks lead.

Of course, on light-skinned game with a chest that may only be 9" to 12" across, the issue is moot, so long as the bullet doesn't just blow up upon contact. Nosler partitions have a somewhat sacrificial lead front, and an encapsulated rear for the best of both worlds.

Traditionally, a S.D. of .300 was recommended for dangerous game (a .375 bullet of 300 grains has .305 S.D., for instance)

We are still learning about the relationships of how bullet shape, hypervelocities, etc., effect penetration, but it's looking like the .300 S.D. figure is still pretty valid.

SD = bullet weight in lbs / bullet diamiter squared.

Penetration in bone = SD x impact velocity. (assuming no deformation)- also assuming that bullet is not too long for diamiter (like the 160grn 6,5mm bullet) For enough penetration on elephant from any angle you need
SD= .320 @ 2150fps or
SD = .3 @ 2300fps

Soft points are a whole different ballgame. and the old CW was that you needed a High SD for large game. ie a 175grn 7mm bullet was better than a 150grn. Today you need to compare bonded with bonded, or regular with same.

Now Now Gentlemen! did you not know that SD is a non entity in ballistics...... Just take a peek at the revival of the SD wars between Gerard, Chris et al.

AR has gone so far as to collectively "invent" a new term for SD Mx / Msa or something to that effect?

http://www.reloadingpro.com/Sectional_Density.htm

quote:

Sectional Density = (Bullet Weight in Grains/7000 grains per pound)/Diameter2. As an example, let's take the .30-caliber 180-grain bullet and work through the formula. Sectional Density = (180/7000)/(0.308^2) = 0.0257 pounds/0.0949 sq. inches = 0.271 pounds/square inch.

For a personal example, my 370-grain North Forks for my .416 Remington Magnum have a .305 sectional density.

Russ

So SD has got nothing to do with the lenght of the bullet but only concerning about the diameter.

High SD bullets are longer than low SD bullets of the same caliber because a longer bullet in any given caliber will weigh more than a shorter one, and weight is one of the two key factors, along with cross-sectional area, in determining SD.

quote:

Originally posted by mrlexma:
High SD bullets are longer than low SD bullets of the same caliber because a longer bullet in any given caliber will weigh more than a shorter one....

True in most cases, but not all. Some years ago, I was looking into tungsten-core bullets for match shooting but, at $7.99 a bullet (at the time), I passed. Load development would be a real costly proposition.

I do like Speer's tungsten-core bullets, though. The main advantage is, of course, that using a shorter bullet allows you to chew up less case capacity where powder is concerned, versus a "standard" bullet (with a lead core -- or a monometal solid) of the same caliber and weight. Having more case capacity available for more powder has its advantages when you're trying to drive a heavy bullet as fast as possible.

I'm not taking issue with your comments, Mr. Lexma, and I hope it doesn't seem that way. For what you stated, you are correct. I just wanted to expand on what you posted.

Had I been the (unfortunate?) winner of the .45-70 Gov't.-mandatory buff' raff' for 2003, I was going to use Speer's tungsten-core slugs and load 'em hot.

Russ

Russell, yes you are right of course. For my statement to be true the bullets being compared must be of the same composition.

Keep it real simple, SD = penitration...I put a lot of stock in SD, much more that velocity or energy...

SD as a means to measure penetration is a poor number. Example a .375 - 300 gr nosler partition (not sure there is such a thing) and a 300 gr monometal solid. Same SD but completely different penetration ability.

As with all of these mental mathamatics a good dose of common sense needs to be applied. SD is a good indicator when talking about bullets with similar construction. A 300 gr bullet will out pentrate a 270 gr bullet of the same design.

JMHO John

quote:

also assuming that bullet is not too long for diamiter

Ganyana,
What stability factor would you peg as the minimum for reliability on DG with respectively a soft and a solid?

Hi Gerald

Don't know about softs- I would "assume" that stability isn't an issue as the expanding nose would stabilise the bullet and negate any effects of 'wobble'

On a solid, bullet stability is, of course, a major factor in bullet penetration. The formula for penetration of a solid in bone given above asummes many "constants"; such as a stable bullet, and which in reality, may not be constant. Some .458's and many .375's have a twist rate such that the bullet is still yawing slightly at normal elephant shooting distances.

For those interested, I recomend Mike la Grange's final work on bullet penetration in elephants. The dissection and analysis of bullet performance on 3200 adult elephant is the sort of work that will never be done again. Sadly Mike lack the most scientific approach to the work, and was heavily influenced by Art Alphin, but the results on the actaul elephants is - Results! The interpretation may be a little different- Art made his bullets blunter, I ordered flat nosed bullets. Art belived that a longer bullet (such as a monolithic) would be stabilised by the flesh and bone better than a short bullet. - I don't belive that made any difference and that launch velocity and twist rate are far more important. Still, as a reference work of hard data, Mikes' work stands alone.

Ganyana!

Where can we get a copy of La Grange's results?

465H&H

Mike La Grange, "Ballistics in Perspective" available for $19.95 from http://www.amazon.com

http://www.amazon.com/exec/obidos/tg/detail/-/096248072...nce&s=books&n=507846

Truely the monometal bullet has changed the course of history in regards to Sectional Density and it has also complicated my life, for which I am eternally grateful and at the same time left, from time to time, in a state of monumental confusion!

Sectional density is directly proportional to the length of the bullet (its a little more complicated by the density of what the bullet is made out of). More accurately, if you squished it to be a cylindar in the barrel, it is proportional to THAT length. Long skinny bullets tend (roughly) to have more penetration than stubby bullets -- except in as much as bullets can come apart -- because they have (and transmit) more momentum per unit area than a larger bullet with same mass and energy.

There are so many other factors in play that people can argue and produce counter examples forever over how important the above is -- which leads to much fun and campfire discussions... or which lighten dry office days chained to a desk :-(

Dan

For a soft, the sectional density that really matters is the sectional density of a bullet after it has finished expanding. A bullet which loses weight during expansion also loses sectional density. Bullets that blow their nose off (Nosler partition) or lose petals (Barnes X, depending on velocity), or lose a lot of lead during expansion (Woodleigh, depending on velocity) suffer a substantial drop in sectional density as a result of expansion. This in turn reduces total penetration depth. If you want deep penetration, a bullet that retains weight and retains its original sectional density even after expansion is a good place to start.

The only value sectional density has is with solids, and then only if they remain in perfect reloadable condition when recovered. The entire calculation becomes worthless the minute a ***Soft point*** bullet impacts the skin.

The calculations would have to be made during the infinate variable expansion process to have value. That won't happen! There are way to many variables upon bullet impact. It's not practical or likely possible to make a formula that could work at determining what that sectional density was capable of providing as a benift.

Now if you're talking solids which don't change shape then you can use sectional density to some degree. However since the bullets diameter and weight determine Sectional Density and those are the two things most greatly affected upon impact with soft points( hunting bullets)the calculation is pretty much worthless!

The formula for S.D is stated above numerous times.
Sectional density is not a ratio of bullet length to caliber. People view the 6.5mm 160 grain bullet an exclaim "that must have a sectional density outta this world" or "look at the S.D. of that bullet". When, in fact the 400gn.416 bullet has nearly identical S.D. The two are as follows .328 compared to .330. The 416 doesn't look as long does it? However, they are very close to the same length. Before ya'll get your panties in a wad, remember, jacket thickness, nose profile etc will cause these two bullets to be of different lengths due to their construction.
As stated above, we need to compare apples to apples. Although I haven't done this yet, I suspect that if you turn cylinders of the same alloy, to the same S.D., regardless of caliber. they will be the same length. The best way I know to describe S.D. is a water column.
If you have two water pipes, one 1 inch in diameter and 50ft. tall and the other 50ft in diameter and 50ft tall, they will have the same water pressure at the bottom. They have the same mass per cross section. So, homogeneous cylinders would be the same length, if made from the same material.
If the bullets are made of a denser material they will be shorter for the same S.D. but the mass is the same.

Does S.D. have anything to do with soft point bullets? Comparing apples to apples, If both bullets are constructed identicly and have the same impact velocity and have the same rate of expansion, then the abrading or wearing properties of the material being penetrated has more material to wear off with the bullet of higher S.D. (not counting retained expansion, we're going to wear this particluar bullet off.) Just go sharpen your pencil in the sharpener sometime. It takes longer to get to the eraser of a new pencil than one of the nubs you dug out of your desk.
For you guys that know more about this than I do. yes, I know there is a mass issue here too in the above scenereo, and resulting expansion, and a list of variables that go on and on.

Just the tip of the iceburg, Hoss. Hope our boat's name ain't the Titannic

Thanks
Todd

Bullet weight / area = density times bullet volume / area = density times area times length (for cylindar) / area = density times length is proportional to length. Quod erat fasciendum.

The point about expansion is correct to the extent that the effective area increases, so you have a much larger volume of material absorbing momentum.

However, while numbers like SD can give a simple picture of things for comparison, there are lots of sources of variability between tissues, bullet construction, etc that can overwhelm any simple picture. Another thing to keep in mind, ballistic coefficient is proportional to sectional density (Hatcher's Notebook is a good source for details).

Dan

.

I have been firing bullets into a variety of media and animals for the last 12 years and observing the terminal behaviour. The pity is that I am not trained as a researcher and record keeping consisted mostly of scribbling on whatever flat surface was available and trying to remember what happened. Stupid of me, but there it is. Up till five years ago I regarded Sd as a factor of importance and then realised that working with Mo/XSA is a far better starting point. For Sd to make any sense, speed must be added into the equation and the instant that is done, it is no longer Sd. Why start back there if you can start over here and save the time and effort?

Look at all the testing members here have done and reported on. Without fail, in every instance, no inference can be drawn from Sd, momentum, energy, speed, bullet weight or calibre, as factors on their own, regarding potential terminal performance.

The closest one could probably come to a handy way of comparing probable terminal performance, across calibres and construction, is Momentum/Cross Sectional Area (Mo/XSA) and then only if the number is considered together with bullet shape, bullet construction and the effect of impact speed on the way the bullet deforms. Same calibre and construction is easy and does not warrant much discussion.

The line of thinking that has eveolved is thus:

Here are three bullets with the same Mo/XSA. One is a solid and the other two are bonded core RN/FB bullets. The solid and one B/C bullet are fired at the same speed and the remaining B/C bullet is fired at higher speed. Mo/XSA assumes identical penetration depths which we know is wrong. So we adjust the probable penetration depths and probable wound channel shapes by adding that the solid will go deepest, but with a narrower wound channel because of the fact that there is no expansion. The slower B/C bullet will go deeper than the faster one, but with a wound channel of lesser volume because of less stagnation pressure, less expansion and less energy. Substitute one B/C bullet with an expanding mono, and you start over again.

Everything is a compromise and, relying on a single indicator such as Sd or muzzle energy to predict terminal performance, is a fallacy.

.

Alf said it all in a few words, that sums it up..and to say SD is not one of the most important factors in maintaining velocity and penitration is just not correct..It is one of the most important factors in bullet making, at least with lead core jacketed bullets of equal construction...

I'm not sure there's SD wars -- it looks like most people agree on most points, though focus on different points.

1) Sectional density is a simple lumped parameter that measures weight per unit area (that ratio is proportional to length). It gives a good sense of how well a bullet is likely to transfer its momentum (whatever that is) into its particular target per unit area (mass * velocity / area = SD * velocity -- with some normalizing constant factors, there's a pi in there, for instance).

2) There are other important factors (target structure, bullet composition, etc). Whether the bullet retains its shape is significant (SD tends to make most sense for generally cylindrical things; when bullets start loosing mass, shart changing shape as the projectile moves through the target, etc, the effective SD starts to change radically). So the question is whether the SD number for an unfired projectile gives you a decent predictive number? Yup -- but with some recognition that the same bullet may behave differently even when fired under nearly identical conditions (well -- one went through the elbow, and the other didn't).

Is there any particular disagreement about any of these ideas?

Dan

Even when comparing similar construction and shape, cross caliber comparisons reliant largely on Sd do not work. The testing done by 470Mbogo and shown in the chart below is interesting proof of this. The first eight points on the chart all represent monometal round nose solids of different calibres, weights and speeds. The last two are jacketed lead solids. Here we see that bullet construction and shape is by far the most important factor governing penetration, albeit in a wood and air sandwich.

MacPherson makes much of the bullet form factor and this is something I found to be very true. The shape of the bullet after impact and deformation (or not) is far more important than any other single factor I can think of. Ray has made this point on a number of occasions where he observed that bullets that mushroomed beautifully and retained virtually 100% weight, lacked the qualities that are required to create an effective wound channel.

Consider the points on the two charts below where, with solids of differing construction, a greater disparity is proven than with either Sd or Mo/XSA. One thing is clear to me though, and that is the fact that Sd does not follow the penetration plot, while Mo/XSA does and only shows a deviation when a different bullet type enters the equation. We must also not forget that penetration depth is only one of three important factors in the wounding mechanism.




DanEP said correctly: "Long skinny bullets tend (roughly) to have more penetration than stubby bullets -- except in as much as bullets can come apart -- because they have (and transmit) more momentum per unit area than a larger bullet with same mass and energy." What he describes is Mo/XSA and not Sd. He also said: "However, while numbers like SD can give a simple picture of things for comparison, there are lots of sources of variability between tissues, bullet construction, etc that can overwhelm any simple picture." I have to agree.