Home Forums Hunting & Shooting General Big Game understanding difference between sectional density and caliber

Though I've been shooting for over 30 years, I find my mid-life
years full of more learning and curiosity for information.
So one of the topics is S.D. vs. caliber size and weight.
More specifically, I shoot 30-06 and have compared the S.D,
of the 165 and 180 gr. to smaller calibers like 25 cal, 6.5, 270, and 7mm. which in most cases have higher density than the
bigger and heavier 30-06.
With medium game it's not as big a deal, but lets consider
elk and moose size hunting. Am I going to see better killing
potential with say a 270 150gr. or 280 160gr. having S.D.s
of .279 and .283, or the slightly wider and heavier 165 or 180gr. 30-06 with S.D.s of only .248 and .271?
And the other interesting fact is all of these caliber's have
very similar ftlbs of energy at 100-200 yds, so about the same
"hitting power", but differing weights and densities.

Hoping this makes an interesting and informative post.

Aceman.

Use a TSX and it won't matter which you use

SD usually implies a higher BC, meaning better exterior ballistics at long-range, but the most important thing that comes from a high SD is straight and deep penetration. The bullet can expand further back on the shank before it becomes a "pancake".

Sectional Density (SD) boils down to the length and weight of the bullet in relation to expansion of the bullet during penetration. The longer and heavier the bullet the more time it has for expansion and the slower it expands the more penetration you get because your pushing a smaller surface, thus you get more penetration as it expands slower..The cross section of the expanding bullet slows the bullet down and stops penetration therefore the slower it expands the more penetration you get..

I suppose you could and should add to this scenario that the longer a bullet is the better it holds its velocity at longer ranges all things even steven..so a longer, heavier bullet of the same shape will normally shoot flater over very extended ranges because it holds its velocity better than the lighter bullets that lose velocity quicker..Thus supposedly, a 200 gr. 30-06 starting at 2600 FPS will maintain velocity better than a 150 gr. starting at 2900 FPS over say a 600 yard range, therefore the 200 gr. bullet will shoot flatter at extended ranges.

This is an over simplification, but it about says it all..Keep in mind that penetration is the first and foremost important aspect of any bullet because if it does not get in there and do its job then it will fail.

Folks can get totaly anal over these things, but in the real world, most of it is just entertainment for young men that are eaten up with technicalities of balistics, love to discuss the pros and cons, and it has far reaching serpintine arms of confusion and myth, and it takes years to come back down to reality and admit its probably all BS to start with, but it was a fun trip..:)

how is SD applied when you shoot 'em in the neck?

Sectional density is simply the ratio of the weight of the bullet to the cross sectional area of the bullet. Thus, all bullets of the same caliber and weight will have the same sectional density. Given bullets of the same caliber but different weights, the heavier the bullet the greater the sectional density. Given bullets of different calibers but the same weight, the smaller caliber bullets will have the greater sectional density. Sectional density is probably only valid as a predictor of performance when applied to bullets of the same construction and form.

Ballistic coefficient is used to predict ballistic performance and is a product of a number of aspects of a bullet's form which affect the way the bullet moves through air. Given two bullets which have the same form (same caliber, same ogive, same base) but different weights, the heavier bullet will have (incidentally) a greater sectional density (because it's heavier) and a higher ballistic coefficient (because it's heavier and longer.)

Ace,

I look at SD & BC like this:

SD describes the penetration characteristics of a bullet. So, the higher the SD the better the penetration theoretically.

BC describes the flight characteristics of a bullet. Meaning, the greater the BC value the flatter it will shoot and it will experience less wind drift.

I look at it this way, because the math behind simply gives me a headache.

This is why many shooters like to use heavy for caliber bullets like 200gr .30 cal, 175gr .284 cal, 140gr .264 cal, especially if you live and hunt in places where you can shoot a long way.

CLB

i say keep shooting your 30-06 (with a good bullet) and don't sweat the small stuff. no telling how many elk/moose (and everything else on the planet) have been taken with a 30-06. while there clearly IS a difference in sd and bc between bullets, at the "usual" hunting ranges, neither matter a great deal.

you leave out the fact that the .30 caliber bullets do not stop at 165 grains... the highest sectional density commonly available in a hunting bullet is the 220 grain .308 bullet...

if construction is similar, a higher sectional density may indicate the ability to penetrate deeper

sectional density is simply the relationship of weight and diameter... the sectional density of any diameter bullet can be increased by increasing its weight...

many newer mono-metal bullets made of copper or gilding metal rely far less on sectional density to achieve desired penetration, than standard jacketed lead bullets...

around the time that my oldest child was born i loaded some .308/220 monolithic solids for my 30-06...
just in case i needed to drop one of those rogue northern illinois elephants, ya know...

Ringworm,
With good SD you get two holes, with bad SD you get one hole, can't get any plainer n that. With real bad SD you get a blow up on the skin and I have seen this more than once.

SD is irrelevent to expanding bullets and construction can blow holes in any argument.

An 85gn TSX will out penetrate a 100gn Cup and Core 6mm even though the SD is higher with teh heavier weight. This repeats right up teh line to any caliber where a lighter weight (shorter) bullets has superior integrity of design.

The whole argument is pointless unless you are talking solids and dangerous game, even then, mono metal Vs Steel jacket Vs naval bronze Vs round point Vs flat point and on it goes.

JW

SD is a function of bullet weight and non-expanded diameter. It is not a measure of terminal performance, such as shooting them in the neck. Any two bullets with identical diameters and identical weights will have identical SDs regardless of their construction or how they penetrate.

Having said that, bullets of higher SD do tend to penetrate deeper in game animals for two reasons: 1- Higher SD means greater weight, which means lower velocity, which means slower and/or less overall expansion, drag, and "hydraulic effect"; 2- Greater weight means more momentum.

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That really depends on the construction of the bullet...

Because with an expanding bullet, the cross sectional area and weight (usually) change, thus SD changes during terminal ballistics.

There are a couple of guys down the road from here who seem to kill a lot of deer and I don't think either one could define sectional density if it were sitting on his kitchen table. Man, you just have to wonder how some of these guys get by in this world...

I respectfully disagree.

SD does count - with expanding bullets, as well as solids. The only "if" is - IF the bullets are of the same type. Even with an all-copper bullet like the TSX - the one with the higher SD will penetrate further than the one with less SD.

Based on my own back-yard tests (and only comparing bullets of similar brand) the SD number does give one a pretty good idea of how far the bullet will penetrate - even when comparing bullets of differing speeds and caliber.

Therefore - the arguement, or discussion is anything but pointless.

There is a reason (even when looking at expanding bullets) that the 6.5 Swede with it's long high SD 160 grain bullets, the 7mm Mauser with it's long high SD 175 grain bullets, and the 30-06 with it's long high SD 220 grain bullets developed such a great reputation on killing game larger than their caliber might first indicate.

It was because of the great SD these bullets all carried - even though they were all designed to expand.

I do however agree that it would be a meaningless number when comparing bullets with different expansion characteristics. SD is only useful when looking at bullets of similar construction - whether they be soft-points, hard-points or expanding all-coppers.

Thanks for the replies on S.D. and it seems to be a combination
between good penetration and controlled expansion. But what about a bullet with maximum penetration and high S.D. without good expansion?
All the respected gun writers agree that expansion along with
penetration is what kills, creating a wound channel causing
internal devastation of tissue, as long as you get to the vitals.
So do I trade some expansion for more penetration and higher S.D.
or a little less S.D. and penetration, for good controlled expansion?

Aceman.

aceman,
You make that judgment by what animal your shooting, mostly on size..On a Moose or Alaskan bear I want SD and its penetration. On a deer or pronghorn antelope, I'll take expansion..but moderation is the word for success, and just too many folks go to the extreme in either or both directions..

SD, velocity, penetration, expansion and a host of other coined terms boil down to one component and its the one that works best.."common since".

And that statement explains exactly why bullet construction also matters. A good controlled expansion bullet retains its mass with limited increase in diameter (i.e. it retains its SD) while a varmit bullet will lose much of its mass while expanding drastically.

Do you think those guys could explain how gunpowder is manufactured? Does that make it more or less important?