Sectional density

[The Sodbuster]

If I understand correctly (and correct me if I'm wrong), sectional density relates to a bullet's length relative to its diameter.  6.5mm bullets have a reputation for excellent sectional density, and therefore excellent penetration and good downrange performance.  

But couldn't you choose a heavy enough bullet in any caliber and get the sectional density equal to a 140 grain 6.5mm bullet?  I don't have anything against the caliber (in fact I'd love to get a 6.5X55 or .260 Rem), but I don't see anything unique about 6.5 mm bullets having a monopoly on great sectional density.

[Don Fischer]

The answer is maybe. I think to equal the sectional density of a 140 gr 6.5 bullet you really have to get big for caliber. But sectional density by itself is just a number giving an indication of the bullet's ability to penetrate.

Without velocity, there's no penetration and I've read a report where to much velocity will also retard penetration. Take a 180 gr 30 cal bullet and drive it out of a 300 Savage and the same bullet out of a 300 Weatherby and there will be a difference in penetration because of velocity difference.

Then if you start two bullet's of the same sectional density at the same velocity but one of higher ballistic coefficent, the higher ballistic coefficent will arive down range at higher velocity and in theory penetrate deeper. I say theory as if the don't both meet the same reistence in penetrating, then restence will play a role slowing down the bullet and reducing penetration.

Then too, bullet construction enters the picture. Take two bullets of like sectional density but different design, say a 55 gr 224cal where one is a soft point and one a FMJ, and the FMJ should penetrate deeper if for no other reason than it will retain 100% of it's weight. Think of the difference between a golf ball and a ping pong ball. They are about the same size but the weight of the golf ball carries it farther with equal starting velocity and resistence.

The X type bullet's penetrate deeply thanks in no small part to the way they retain weight. The flaw in them, and this is opinion, is the small hollow point tip on them. You'll also find that tip on ULD type bullets. Sometime's those small tip's close up on impact rather than opening. The one's I've seen do this I intentionally fired into a medimun to see what happen's to bullets and check weight retention. This year Winchester put a poly tip on their "Fail Safe" bullet and I understand that Barnes is doing the same with the "X" bullet. I don't know that either said why but I expect it's to promote reliable opening of the tip. Read about bullet failure in the Africa thread by JJ Hack.

The fact that so many people swear by the "X" type bullet's is I believe because game generally shot in this country is relatively light skinned, light muscled and light boned. Let's face it, a deer, is not all that tuff an animal neither is an Elk when shot broadside into the lung's.

Personnally I believe that the best way to select a bullet is to choose a medimun to heavy for caliber bullet. You won't find sectional densities noted on factory ammo boxes. In 24 cal, the 100 gr seems to be the weight for hunting game. In 25 cal it's 100gr and up, 6.5 is 120 gr and up and so it goes. That people have had good luck with lighter bullet's in the different calibers is more an indication that they are giving a lot of thought to where the bullet is placed and avoid certain shot's. You'll find that some bullet maker's are making various bullet's in lighter weight's but of different construction to make up the difference.

Thing's like sectional density and ballistic coeficent are fun to have discussions about but we've been killing animals for years without any knowledge of these thing's.

[nasem]

sectional density = bullet diameter * bullet weight * something else i forgot, its not bullet length

[nomosendero]

That is correct, it is the weight of the bullet relative to the caliber or diameter of the bullet. For example, a 30 cal. 180Gr. X bullet has the same Sectional Density of a 180Gr. Sierra even though the X bullet is much longer. Same is true of a round nose versus spitzer versus boat tail,
etc., if it is the same weight & same caliber, it will have the same Sectional
Density.
The other 2 factors that influence penetration are the bullet construction &
the velocity at point of impact. Concerning bullet construction, the small
point in early X bullet designs gave some expansion problems but I have never heard of this same problem with the newer TSX bullet with American Game.
Concerning the Poly tip that will be on the Win. bullet & the Syn. Tip for
Barnes, both companies have stated why they added this & BTW the MRX Barnes will NOT replace the excellant TSX. Both companies have stated that the tip will allow the bullet to expand at a lower velocity, Barnes claims about 200 FPS, which is nothing new as the poly tip offers the same advantage over the soft point or conventional lead tip. The other reason for the tip is to increase the BC & this allows for a better long range bullet, again nothing new. Concerning the VLD or ULD bullets, many
will not expand well, but some made for hunting do & Wildcat Bullets makes several of those & I have found out through actual use on animals
that they do. Reports are coming in now that the Wildcat will even expand on small animals like Coyotes at long range.
Sodbuster, the other question you asked  was can you get this same Sec.
Den. with other calibers? Well, yes & no.  You can get this much S.D. with
heavier offerings & still use the factory twist rate like a 30 cal. 200 Gr. which has a SD of .301 as opposed to a SD of .287 for the 140 6.5 or
.283 for 7mm 160 Gr.(very close) or .310 for the 175 Gr. 7mm. The 338 cal. 250 Gr. has a SD of .313 In 25 cal., I get close to the 6.5 SD with the 125 Gr. Wildcat bullet. But in some calibers like the 6MM, 22, 35 & others,
you will need a faster twist barrel with custom bullets/barrels  to obtain this & so it depends on caliber & whether factory or custom.
 
However the different lengths & shapes of bullets of the same weight will
dramatically change the BC of the bullet.

[safetysheriff]

sectional density (sd') is the correlation of a bullets weight (mass) to its diameter.

to combine higher sd' with reduced recoil calls for a relatively heavy bullet in a caliber of .270 Win' or less in my opinion.....

bullets of higher sectional density usually travel at a lower velocity from a given caliber/cartridge than do bullets of a lower sd'.   they will, therefore, usually have a lower impact velocity at 'standard' target/hunting ranges.....such as out to 300 to 350 yds' maybe.    i am not speaking about the new 'managed recoil' iterations at this point!    lower impact velocities reduce the stress that is placed upon the jacket and core of any given projectile........if the bullet in question is equipped with a core.    

bullets of higher sd' oftentimes are designed for greater penetration than are those of lesser sd'.  

you need to do a 'search' on the web to see what else you can learn about sectional density.   in the years gone by, up to maybe 7 to 10 years ago, sd' was often an indicator of what a bullet was intended to accomplish as regards its terminal ballistics.    low sd's were for quick expansion, high sd's were for greater penetration and heavier game.   some of that has definitely gone out the window more recently.

ss'

[Slamfire]

Oops!  :roll:

[Slamfire]

Yes you could, trouble is there aren't many calibers that have heavy enough bullets to match the 6.5mms 160 grain bullet. With a sectional density of .328 it is not matched by any .375, .358, .338, .323, or 7mm bullets. Only the 220 grain .308 bullet at .331 exceeds it.
Given the same construction, having the same impact velocity, into the same medium, the greater sectional density will show the most penetration.
The thing that makes the middle grade 6.5mms and 7mms kill so well is the moderate velocity, that delays expansion until penetration into the vitals has occurred.  :wink:

[grousehunter]

just make your 1st one count, and the rest are only for the EGO!!!!!!!!

[Iowegan]

The formula for sectional density is: A bullet's weight in pounds, divided by the square of its diameter, in inches. In the 140gr 6.5 bullet, 140/7000=.02; .264 squared=.06969; .02/.06969=.287

Most reloading manuals published by bullet makers (Speer, Hornady, etc) will list the sectional density of their bullets. The original 6.5x55 used a 140 gr bullet. All 6.5mm bullets that weigh 140 gr will have a sectional density of .287 (very high).

The only bullets that beat a 140gr 6.5mm for sectional density are 7mm bullets that weigh 165 gr or more, .308 cal in weights of 200 gr or more, 8mm in 210 gr or more, .338 above 230 gr, .375 weighing 285 gr or more, .416 of 350 gr or more and a .458 in 400 gr or more.

Sectional density is a good attribute for penetration, however as stated before, the velocity and construction of the bullet can limit penetration if it expands (mushrooms).

[Coyote Hunter]

...
But couldn't you choose a heavy enough bullet in any caliber and get the sectional density equal to a 140 grain 6.5mm bullet?  I don't have anything against the caliber (in fact I'd love to get a 6.5X55 or .260 Rem), but I don't see anything unique about 6.5 mm bullets having a monopoly on great sectional density.

They don’t.

Various S.D.s:

.287 = 140g, 6.5mm
.328 = 160g, 6.5mm

.283 = 160g, 7mm
.310 = 175g, 7mm

.271 = 180g, .308
.301 = 200g, .308
.331 = 220g, .308

.313 = 250g, .338

.286 = 420g, .458
.313 = 460g, .458
.340 = 500g, .458
.367 = 540g, .458

[Zachary]

I

But couldn't you choose a heavy enough bullet in any caliber and get the sectional density equal to a 140 grain 6.5mm bullet?

Yes, of course.

I don't have the actual numbers in front of me, but let's assume that the SD of a 140 grain 6.5mm bullet is the same as a 200 or 220 grain .308 bullet (whether .30-06, .300 Win Mag, etc.).  Okay, so the 6.5mm/140 grainer does not have the monopoly on SD.  HOWEVER, the recoil from a .30-06 with 22o grainers is going to be much more severe than that of a 6.5mm/140 grainer.

Also, keep in mind the practicability of using a .30-06/220 grainer - are you going to hunting elephants?

You may find it difficult to believe, but in the Scandanavian countries, hunters use the 6.5mm/140 combo to successfully havrvest Moose every year!  Heck, in the US we would think that you need a .30-06 with stout 180 grain bullets.  Of course shot placement is everything, but there is a lot to be said about using a relatively light 140 grain bullet with high SD over a .30-06 with 180 grain bullets.

Also, when shooting thin skinned game like whitetails, you really don't need THAT much penetrations.  Sure, it is nice to have a bullet fully penetrate a deer so that you can have (if you want to) two holes to aid you in following a blood trail.  However, you don't need a bullet to fully penetrate a deer AND THE TREE BEHIND IT.  I would imagine that's what a 30-06 with 220 grainers would do, while a 6.5mm/140 would not.

Zachary

[safetysheriff]

The only bullets that beat a 140gr 6.5mm for sectional density are 7mm bullets that weigh 165 gr or more, .308 cal in weights of 200 gr or more, 8mm in 210 gr or more, .338 above 230 gr, .375 weighing 285 gr or more, .416 of 350 gr or more and a .458 in 400 gr or more.

Sectional density is a good attribute for penetration, however as stated before, the velocity and construction of the bullet can limit penetration if it expands (mushrooms).

i would think that the 170 and 180 gr' bullets sd's for the .270 Win' are significantly enough above the sd' for 140 gr's in a 6.5.....to include their consideration in any discussion of bullets that have decent velocity and yet very heavy sd' .     if you think of a 130 gr' bullet out of a .270 Win having the sd' of approx' 162 gr's in a .308 cal'......and then think of its significantly greater velocity, you can see why some of us are so impressed with Jack O'Connors cartridge.    

ss'

[Iowegan]

safetysheriff, The 270 bullet is known for high sectional density too. The heaviest recommended bullet weight for a 270 Win is 150 grains, which runs about .279 SD. A good number but not quite as good as the 6.5mm. By the way, the 270 Win is one of the very best elk and moose cartridges on the market.

The problem you run into when using bullets that are heavier than normal is the twist rate of the barrel is too slow.  Most times, a heavier bullet can't be driven fast enough to get the spin rate without exceeding max chamber pressure. The 270 Weatherby Mag with its 1:10 twist rate can handle bullets up to 170 gr because it can push them much faster than a 270 Win.

If you look at twist rates for 6.5x55 or a 7mm Mauser, you will typically see 1:8 to 1:9. 270 Win are 1:10 and come in a little too slow for anything heavier than 150 grains.

What determines the twist rate is the length of the bullet. Naturally, the length dictates weight and weight determines max allowable powder charge. So basically, it's all tied together.

In 308 diameter bullets, you can get sectional densities above the 6.5mm if you go above 200 grains. Here again, you can run into a twist rate problem. Fortunately, .308 barrels are available with faster twist rates so they would work quite well.

Bullet spin decays at about the same rate as velocity. At some magic distance down range, the bullet spin will slow down enough where the bullet begins to wobble and then starts to tumble. Of course that means accuracy is kaput and terminal ballistics are history too.

Here's an example for a 270 Win with a factory 150gr bullet. The spin rate will decay and cause instability at about 300 yards. At distances closer than 300 yards, you can take full advantage of the high sectional density. At distances beyond 300 yards, you would need a shorter (thus lighter) bullet to maintain stability.

Interesting note: A 6.5x55 Swede with a 140gr bullet will maintain stability for 1400 meters (about 1500 yards). I find it absolutely amazing that a cartridge invented in the late 1800's is still one of the best long range cartridges ever made.

[Cement Man]

I'm gonna print this thread and install it in my shooting info binder. Lot's of good discussion and information.  Thanks guys!  

[safetysheriff]

I'm gonna print this thread and install it in my shooting info binder. Lot's of good discussion and information.  Thanks guys!  

CM'
the discussion may be good; but some of the 'information' is not.   i would guess that Iowegan believes what he has posted, but some of it is false.

because of "conservation of angular momentum" the 150 gr' bullet out of the .270 Win (like most bullets out there!) will lose its rotational velocity MUCH SLOWER than its forward velocity.  Barnes used to sell an 'Original' bullet that weighed 180 gr's and Jack O'Connor wrote in the 8th edition of the Handloader's Digest that he loaded that bullet to 2650 fps using 54 grs' of (orignal-grade) H 4831 as his propellant.    Try getting that kind of velocity and sd' out of a lot of other cartridges and you'll tear your hair our!   That velocity is roughly equal to the 140 gr' Speer bullet of much lower sd'.    (Speer 13 edition)

the USMC contemplated using the .270 Win' for its snipers in Viet Nam in the mid to latter 1960's.....while my older brother was in Camp Pendleton doing 'paperwork' for them.   They wanted the long-ranged killing power and flat-shooting trajectory but shied away from it because it wasn't a NATO round!  (who cares about NATO  any more?)

if a 140 gr' bullet in the 6.5 mm is supposedly good for 1400 meters, how far will the Sierra 135 gr' MatchKing go before it becomes unstable?   They don't make Match-Grade bullets to lose stability at 300 to 350 yds.  Please believe me!

take care,

ss'

[Cement Man]

SS,
Thanks for your input.  My statement stands - Lots of good discussion and information.  I don't think there ever was a thread, article, book, or otherwise that I quite agreed with everything contained therein, but I selectively pick things up and retain them and that is how I learn and grow.  I've been shooting, hunting, and reading about it for about fifty years and although I profusely admire the Keiths, O'Connors, Hagels, and so many others, I do not completely agree with everything that any of them wrote.  Still, I appreciate the quality of the input in this discussion as well.

[nomosendero]

Cement Man
I agree, afterall there is only one book that I totally agree with.

Being human, even when we have researched & tried to convey the facts,
some personal feelings or opinion here & there will come into play & make
a book or article less than 100% accurate. I think the writings of Jack
O'Connor is a good example, he offered alot of good info, but also had a
great deal of bias. As much as I enjoyed Elmer Keith articles, you could
see much of the same. I think Mr. Hagel was very accurate, but being
human, a few personal thoughts would show up. And some of these personal views are worth reading when conclusions are drawn as a resut
of experience if witout bias.

That is when we need to engage our brain & weigh the info & separate
fact from opinions. Many times we can read what many people have to say about the same subject just like this thread & a pattern of the real
facts will emerge.

[Cement Man]

:toast:

[Slamfire]

Bullet spin decays at about the same rate as velocity. At some magic distance down range, the bullet spin will slow down enough where the bullet begins to wobble and then starts to tumble. Of course that means accuracy is kaput and terminal ballistics are history too.

Can you cite some kind of authority for this concept, for the fellah that taught me. said the rate of spin decayed very slowly, and the reason bullets go unstable way down range was the transition through the sound barrier.

There isn't any magic in the 6.5 mm, its bullets obey the same physical laws as any other caliber. That .270 150 grain bullet ought to hold its stability nearly as long as the 6.5/140.