Gentlemen

I have a discussion with my buddy about pentration related to caliber...bullet weight related to caliber.
We have an unsolved discussion going as:
I say the best relative pentration can be obtained by a bigbore in the 40 caliber range..
My buddy says that the best relative penetration is obtaind in the 6mm or 7mm caliber range.
I see his point....please advice..

//voyager

What is "relative penetration"?, could be incest.
Good Luck!

Voyager, family issues aside the matter is resolved by what may be referred to as retained SD(my non-scientific term). As a rule of thumb SD may be viewed as an indicator of a bullet's POTENTIAL for penetration, the higher the number, the higher the potential. However it is of great importance that you realize that as soon as a bullet begins to expand that index changes inversely to diameter increase.

The argument in regards to caliber is pointless in any case. More to the point is what the bullet does after impact. One that does not expand or tumble will have a leg up in the penetration contest. Bullet nose design also has a large influence in this matter as well. A bullet that does not expand will also leave a lessor wound channel so precise placement becomes paramount.

It is common to see higher SDs in large bore guns yet some very high figures can be found in selected calibers such as 6.5mm, 7mm, .30 cal, and .338 cal as well. For a specific caliber and form, a higher SD carries a requirement for a faster twist, so there is a practical limit to how far the theory can be carried. Too, longer bullets are more likely to tumble in the terminal phase, so again the matter of the retained SD or resultant SD if you prefer, is likely to be lower.

Bullet construction is far more important than sectional density will ever be.A bullet such as the failsafe or barnes x will penetrate much better than much heavier conventional bullets despite having a lower section density.

I've always considered sectional density one of those "all other things being equal" deals; a rough estimate of penetrating ability that's useful for comparing bullets of different calibers. I agree with the stubblejumper - bullet construction is far more important because "all other things" are almost never equal.

DD & sj are both right. A bullet like the 'X' maintains it's longer bearing surface, SD, during penetration. Even if the bullet max. expands the long for caliber shank pushes it along @ it's more likely to penetrate in a straight line. This is true of all bullets designed to maintain the 50-60% of thebullets shank. A soft long for caliber bullet has a very high SD, but if ti expands & looses it's stabalizing base, plus the great frontal area, expansion would decrease.
If you took 2 bullets of exactly the same design & SD but diff. calibers & drive them @ the same vel., I would bet the smaller diameter would penetrate further due to the smaller frontal area. Not much farther, but farther.

quote:

Originally posted by rxmoore:
I've always considered sectional density one of those "all other things being equal" deals; a rough estimate of penetrating ability that's useful for comparing bullets of different calibers. I agree with the stubblejumper - bullet construction is far more important because "all other things" are almost never equal.

I believe this statement means that solid bullets of equal SD which strike a specific substance, and which are travelling at the same velocity, will penetrate the same distance in that substance if both have sufficient stability to remain traveling point forward. This would be true regardless of caliber.

However, one might argue that the larger-diameter projectile in this test, having a nose of greater cross-sectional area, might be stopped sooner due to greater friction in passing through the substance. If this proved to be true, then the smaller caliber would possess an advantage, if all one was seeking was maximum penetration.

I suppose this idea is one that would require an actual test to prove or disprove.

quote:

However, one might argue that the larger-diameter projectile in this test, having a nose of greater cross-sectional area, might be stopped sooner due to greater friction in passing through the substance. If this proved to be true, then the smaller caliber would possess an advantage, if all one was seeking was maximum penetration.

But you have to consider "momentum." The one with a greater SD must also be heavier. The extra weight helps the heavier one offset the difference in frontal area. Also, the comparisons can only be accurate when similarly constructed bullets are used. For example, one can use two identical NOS Partition bullets of different SD's, but of the same caliber.

The same bullet type for the same caliber may not always be of the same (identical) design. For example, some of the Partition bullets designed to Lazzerony's specifications are different that the standard Partition.

Of two bullets that are of identical construction, the one with the greatest SD has the potential for deeper penetration.

Sectional density is the relationship between bullet weight and bullet diameter. You can figure it out by dividing the bullet weight in pounds (divide the grains by 7000) by the square of the bore.
For example, a 140gr bullet in .284: 140/7000=.02
.284x.284=.080686
.02/.080686=.248
Sectional density is .248 (just about ideal for deer in this caliber)

An easier way is to simply go to SST's Rifle Room and use the Sectional Density Calculator.


Everything else being equal, the higher the sectional density, the better the penetration.

quote:

The one with a greater SD must also be heavier.

Ray, that isn't perxactly correct. See SST's post above.

SST, you are correct, but an expanding bullet looses weight & expands, thus changing it's SD along the way. If that same 140gr bullet expands to .60 & looses half it's weight, what is the SD? A more appropriate way to measure would be to recover the bullets, measure & way them, that is your true SD. That's why todays mon bullets & premium penetrate beyond their SD numbers.

quote:

SST, you are correct, but an expanding bullet looses weight & expands, thus changing it's SD along the way. If that same 140gr bullet expands to .60 & looses half it's weight, what is the SD? A more appropriate way to measure would be to recover the bullets, measure & way them, that is your true SD. That's why todays mon bullets & premium penetrate beyond their SD numbers.

Exactly,that is why bullet construction means so much more than sectional density.

quote:

Originally posted by DigitalDan:

quote:

The one with a greater SD must also be heavier.

Ray, that isn't perxactly correct. See SST's post above.

I stand corrected. What I meant to say was that of two similarly constructed bullets for the same caliber, the one with a greater SD is heavier. There are exceptions, but in general that rule applies to most calibers.

However, there are smaller calibers that have bullets with greater SD than larger calibers, up to a point or course, because as "most" (not all) of the larger caliber bullets get heavier, so does their SD. You can see some of these figures in the tables below:

http://www.chuckhawks.com/sd.htm

Please note that the writer does not include the heavier bullets for the larger calibers (.338, .375, .458). For example, there are no 275-grain and 300-grain .338-caliber bullets. These .338 bullets have quite a high SD over other .338 bullets.

bullet construction (material, shape, bonded/etc)
speed
then SD...

all the other cutsy forumlas work from the last two, btw...


If you are comparing similar bullets, at comprable speeds, the higer SD bullet will go deeper,

ONLY AS COMPARED TO OTHER BULLETS OF ABOUT THE SAME CALIBER

if you compar a 7mm to a .458 bullet, you'll find the 458 will go deeper, due to contruction.

there's no commerical HUNTING bullet that matches the .458 510gr bullet for SD that can fit in the action the round was designed to fit.

here's the deal...

medium/small bores are considered to be high SD if they are .250 or higher

the highest commerical SD 7mm is the 170gr.. sd .301

the .458 510gr is SD of .347

the NORMAL 7mm bullet is a 140gr, sd of .248

the NORMAL 416 bullet is 400 gr sd of .330

the NORMAL 308 bullet is 150 (sd .226) or 180gr sd of .271

the NORMAL HEAVY 6mm bullet is 105gr
sd .254..


the normal .375 is 300 gr sd of .305


your friend is incorrect

even the lowly 405 remington bullet, in .458, is sd of .276

jeffe

quote:

Originally posted by Ray:

quote:

However, one might argue that the larger-diameter projectile in this test, having a nose of greater cross-sectional area, might be stopped sooner due to greater friction in passing through the substance. If this proved to be true, then the smaller caliber would possess an advantage, if all one was seeking was maximum penetration.

But you have to consider "momentum." The one with a greater SD must also be heavier. The extra weight helps the heavier one offset the difference in frontal area. Also, the comparisons can only be accurate when similarly constructed bullets are used. For example, one can use two identical NOS Partition bullets of different SD's, but of the same caliber.

The same bullet type for the same caliber may not always be of the same (identical) design. For example, some of the Partition bullets designed to Lazzerony's specifications are different that the standard Partition.

Of two bullets that are of identical construction, the one with the greatest SD has the potential for deeper penetration.

The one with the greater sectional density would be heavier, IF both are the same diameter. A bullet can have a greater sectional density than a heavier bullet of a larger diameter though. But what I was discussing was two bullets of different diameters that have the same sectional density, for example, a .338 bullet of 250 grains weight has an SD of .31261, as does a .284" bullet of 176.49 grains, or a 6.5mm bullet of 152.51 grains.....

There is a real simple way to settle this issue: Take 20 water filled one gallon milk jugs out to the range. Line up 10 in a row and shoot them, each of you using your favorite caliber and bullet. Then count the jugs that were penetrated, that should resolve the best penetration for those two bullets and calibers at the distance selected.
No theory or degrees required, just hard physical facts...Rusty.

SST has it right. The variables are bullet construction and velocity that controls momentum since weight is a factor of SD. Simply defined, it is the ratio of length and weight to diameter. The effective difference is, at what kinetic energy that happens and is displaced into the target.

I was merely trying to give voyager a simple explanation of sectional density. I still beleive that sectional density matters, and should be taken into consideration when the selecting the proper bullet weight for the intended game when using bullets of conventional lead core construction. However, it is important to note that better bullet construction allows bullets to behave as if they possessed a higher sectional density.