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

Two good posts;Ray's ,too.They illustrate why numbers don't tell the whole story....they are just the start,a reference and not much else.You really can't pick a BG bullet by just reading ballistic tables.

I don't know, I tried to talk to them about burning rate once and they just looked at me like I got off a spaceship from China. And they keep right on killing deer.

Brian,
Thank you for the courtious dissagreement.

Let me explain this another way:

We all read the reviews in RIFLE when the 130 grain TTSX .308 caliber bullet was introduced and trialed in Namibia, with a range of rifles and velocities and saw the penetration reported on game up to Kudu. No surprise there.

We all agree that the SD of any 130gn .308 bullets would be considered inadequate for plains game if using conventional bullet construction. This means that the penetration acheived, irrespective of SD for the aforementioned bullet would compare favorably with a conventional bullet of much heavier weight. It therefore negates SD from the comparison wouldn't you say?

On another note, when I was an SCI President, I met a hunter with International experience that used as his standard load, a .300 Winchester Magnum with a 130gn X bullet. He claimed he never recovered one and never found a need for a heavier bullet.

JW

FYI, I break all my own rules when I select bullets for different cartridges. I would load a 220gn Woodleigh Weldcore in a .30/06 but never load one in a .300 Magnum. I would load a 130gn TTSX in a .300 Winchester but not a .30/06. It has to do with where I would use the rifle and on what game, rather than any traditional logic.

No. Each bullet has a sectional density, inside the animal or not. The notion that high sectional density aides penetration is still a truism, expanded bullet or not. I think you may be hung up on the sectional density given for each unexpanded bullet. You understand that the condition of the bullet changes upon impact and are throwing out the "baby with the bathwater", so to say. A thirty cal TSX bullet of 180 grains that has expanded to 0.85 inches inside the animal has a higher sectional density than a 180 grain TSX that has expanded to 1 inch, thus will penetrate more deeply. SD still is a measurable factor in determining penetration.

Good day.

I was trying not to be picky/anal on this subject, but it may be informative to clear up some of the "sloppiness" in this thread. Most of you guys know this stuff, but it may cause some confusion to use the wrong terms. "Caliber" is not used to determine SD. Bullet diameter is used. A .308 Winchester is a 30 caliber, which is actually 100 times the bore diameter, without the label of "inches." The diameter of the bullet is closer to groove diameter; 0.308 inches in the case of the .308 Winchester with most jacketed bullets.

Also, the "cross sectional area" of the bullet is not used in the equation, although I think it would be more useful than the simple use of diameter squared, which is used. If the cross sectional area were used, the equation would be:
Bullet weight in pounds/cross sectional area in square inches = SD = wt/Πr2 (dang superscript doesn't work for me), or Sectional Density equals weight/pi x squared radius. However, the true equation for Sectional density is the weight of the bullet in pounds divided by the square of the bullet's diameter, in inches; or wt/squared diameter. The squared diameter is actually the area of a square, not a circle, mushroom, x-bullet, etc.. The length of the square's sides is equal to the diameter of the bullet; the expanded diameter or not.

Whew! Got that off my chest!

I wanted to thank everyone for the comments on trying to
understand how S.D. works with bullet weight and diameter.
It would appear that S.D. is important for bigger game, but
not the only element in the formula. But after doing some
comparisons, I find that it's not always that the heavier
and bigger diameter gives you the highest S.D.
A .25 cal. 120gr. is smaller than the 6.5,270 and 7mm bullets,
but has a S.D. of .260, but yet the bigger and even heavier
270 130gr has only .242, the bigger 7mm 120 & 140gr has only
.213 and .248. Even the 6.5 at 120gr has a .260 S.D.?
So if I were to hunt elk or moose, would I have more killing
ability shooting in the some spot with say a 25-06 120gr with a
higher .260 S.D. or a bigger bullet like a 270 130gr with a
lower .242 S.D. or a 7mm 140gr. with a .248 S.D.?
My concern is when do we trade bullet weight and size with a
lower S.D., for a lighter bullet with a higher S.D.?
This is only a concern with bigger game like elk or moose at
medium range, not deer etc., since I'm trying to find a second
rifle caliber that will give moderate recoil, but anchor deer
size to moose. Since a .243 will take all deer size game out
to 200-300yds. with mild recoil, but doesn't give me total
confidence if I encounter bigger game.


Thanks, Earl.

Interesting post as I'm also looking for a second rifle and
caliber that is somewhere between the 243 and 308. I want mild tomedium recoil comfortable for my young boys, but can handle elk and moose at close range if the opportunity came up.
I've been using the 30-06 for years in both 165 and 180gr
partition, but don't want something too close to it.
The 243 is light and easy to shoot, but not totally confident
if we came upon larger game, 308, 270 and 280 seems too close
to the 06 especially in recoil. So I'm looking at 25-06, 6.5's
260 or 7mm-08, but I don't reload so rifle and bullet choice
is very important. Now this S.D. topic may play a part since
25's and 6.5's have as much or more than the slightly bigger
bullets, so for all round shooting from deer to elk with one
bullet weight will I have more ability with a 115-120 gr. 25cal
or a 140gr 7mm-08 partition, since they have similar S.D.'s
but the 7mm is slighty heavier and bigger cal.?

elderstatesman,

If you go with a .25-06 and use 100 TTSX you will just about do everything with it you .30-06 will do. You get about four feet of penetration in elk with that bullet.

There is a post made yesterday why I ordered a .257 Weatherby. It addresses this caliber and bullet.

Of the calibers and bullets mentioned,I'd be inclined to take the 7/08 and a well constructed 140 gr bullet.With bullets being so close in weight and caliber,bullet construction will be far more important than SD numbers on a chart.

Mr. Atkinson,


Me thinks you are missing the argument posted by the majority here, that it's SD AND BULLET CONSTRUCTION that must be taken into account. As you so eloquently posted above. Construction matters as much as SD. I believe it to be 50-50.

It would otherwise be like comparing apples to oranges. But for the most part I think we all are on the same page. SD and CONST. vs SD and CONST. Apples to apples. NO more......no less. Give some of us the credit and ability to know the difference from one SD bullet, amongst the many different types of bullet construction. I go for SD every time with a properly sized for game bullet construction every time. Has worked for over
30 years. But I'm not saying I can't still learn something new.

I would not compare a Sierra Match King with a Nosler Part.
Nor do I believe would you.




Best regards, Bob