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Shooting & Reloading - Mausers, Big Bores and others >> Big Bore Rifles

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SafariHunt
.333 member


Reged: 02/01/03
Posts: 468
Loc: Pretoria RSA
SD Sectional Density ???
      #34843 - 19/07/05 06:18 PM

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.

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"Sleeping under the African sky I can see nothing wrong with this world!"


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Gadge
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Reged: 02/05/04
Posts: 130
Loc: Gippsland, Victoria, Australia
Re: SD Sectional Density ??? [Re: SafariHunt]
      #34846 - 19/07/05 08:00 PM

It's actually a roughish [inexact due to absence of some arithmetical factors, and odd units of measurement] expression of the weight/cross-sectional area ratio.

Obtained by dividing the bullet weight [lbs] by square of diameter [inches].

So for a given diameter, a heavier bullet will be longer [natch] and have a higher SD. And lose velocity [and hence energy] less over distance, than a lower SD bullet.

It's used really as a rough approximation for Ballistic Coefficient, which has to be measured as change in velocity with distance. And is dependent on a lot of external variables like starting velocity, air density, yada yada. This one expresses the bullet's drag function, as a proportion of that of a particular standard test bullet. So nose shape is taken into account here.

Any of the ballistics software packages around can calculate SD. You can do it on a calculator, as Bullet Wt [grs]/(7000xDiam^2[in])



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Cheers,
Doug


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4seventy
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Reged: 07/05/03
Posts: 2210
Loc: Queensland Australia
Re: SD Sectional Density ??? [Re: SafariHunt]
      #34849 - 19/07/05 10:20 PM

I gotta say that for me sectional density does not mean a real lot.
In reality SD is only related to two things, bullet diameter and bullet weight.
SD tells nothing about bullet construction and it does not even tell anything about the bullet shape.
Also the SD number or value cannot be counted on to give an indication of the relationship of the length of a bullet compared to its diameter when looking at different caliber bullets even when they are constructed in similar ways.

For example, if a bullet in a certain caliber had a SD of say .331 and was long in relation to its diameter it is often assumed that a bullet in another caliber with similar SD value should have a somewhat similar diameter to weight relationship provided both bullets were constructed in a similar way.

This is NOT the case however!
To prove the point take a look at a (Woodleigh) 220 grain .308 cal full metal jacket "solid".
The sectional density of this bullet will be .331.
This is a very LONG for caliber bullet where the length is four and a half times the diameter.
Now look at a (Woodleigh) 900 grain 600 NE solid.
This is a very SHORT bullet where the length is only approx two times the diameter.
These are two very different bullets where the 220/.308 is very long for caliber and the 900/.620 (600NE) is very short for caliber.
But guess what.
Their Sectional densities are almost identical!
The 220 grainer has a SD of .331 and the 900 grainer has a SD of .334!
hmmmmm



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Boomer
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Reged: 13/04/05
Posts: 144
Loc: The Hudson Bay Coast, Canada
Re: SD Sectional Density ??? [Re: 4seventy]
      #34861 - 20/07/05 12:46 AM

The situation with soft point bullets as related to SD becomes more compilcated than with solids because the SD value reduces as the bullet expands. There is a very interesting article on this subject at GS Custom Bullets web site and suggests that for the hunter at least, SD is meaningless.

I can't help but think though that long for caliber bullets will kill better than than shorter ones, however I will make some concessions:

1) a heavy for caliber hunting bullet has a long bearing surface relevant to the bore of rifle barrel, and will therefore have a lower velocity at a given pressure than a shorter one.

2) a long - heavy for caliber solid bullet is more prone to failure than a shorter one.

Still - some limited experimentinig I've done with the 380/.375 Rhino seems to support my belief regarding heavy for caliber bullets. I recently compared the 380 Rhino @ 2300 fps to a 270 gr X bullet @ 2800 fps from my .375 Ultra - firing into pails of drill mud. Penatration of the two bullets was dead even, and the volume of the temporary wound cavity was very similar, however, the the permanent wound cavity was much larger for the Rhino bullet than it was for the X. Due to the high impact velocity, the petals had broken off the X bullet leaving a .63" 193 gr "solid", where as the Rhino had expanded to .92" and retained 365 grs. To my way of thinking, the larger frontal area of the expanded 380 gr. Rhino will disrupt more bone and tissue than the expanded X bullet, and allow greater blood loss with less chance of the wound closing. What would happen if the 270 gr X was slowed to the same 2300 fps and the petals had not broken off - I don't know. I do know however that a 300 gr X @ 2600 fps also lost it's petals and performed very much like the 270 gr X. The shank of the heavier X shows signs of bending however, and this may actually reduce it's effectiveness on game.

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atkinson6
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Reged: 26/01/04
Posts: 678
Loc: Idaho
Re: SD Sectional Density ??? [Re: Boomer]
      #35232 - 30/07/05 09:27 AM

SD in conventional bullets of like design means one thing, and that is penitration, and that counts for a lot on big animals...Thats my KISS principle...

Monolithics changed all this to some extent.


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4seventy
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Reged: 07/05/03
Posts: 2210
Loc: Queensland Australia
Re: SD Sectional Density ??? [Re: atkinson6]
      #35244 - 30/07/05 09:49 PM

There really is not much that is too complicated regarding SD.
If you want a higher SD for a particular caliber you just select a heavier bullet and for a lower SD you select a lighter bullet.
Not much rocket science there and about as KISS as you can get.


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