sectionnal density and penetration

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medvedqc

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is there always a correlation between the SD and penetration?

with the advent of mono-metal bullets can we still continue to use heavier bullet or can we use the lighter ones?

bear and cabin fever are coming out .....
 
All things being equal ....yes, but things aren't equal.

The mono-metal such as the X bullet has a performance window down to 2000 ft/s give or take.

If we use a 180gr C&C bullet it will have a bearing surface of x3, a length of x5 and a BC of 0.495, the same 180gr mono-metal will have bearing surface of x3+2, a length of x5+3 and a BC of 0.400. The mono-metal will intrude into the powder space of the cartridge by a value of 3 instead of just x5.

At the same pressure (call it design pressure) the mono-metal will have to be lighter than the C&C so it can be driven faster thus negating some of its poor (comparatively) BC.

So the SD will be less but the lighter bullet can be driven as fast and on game performance (penetration) may be greater than the C&C of greater weight. Some of this penetration comes from a reduced frontal area once the mono-metal expands (the 4 petals have less area than a complete mushroom) cuts like a broadhead.

This is how the X bullet works, this is why weight reduction lower than traditional is suggested (if you require a 180gr then drop to a 150 or 165gr) all in an effort to keep velocities up so the distance can be increased and still have the bullet perform as designed further out.

I have used the 168gr Barnes X and the 200gr NAB in the 30-06 on moose. The 200 has around 25% better SD it has a G7 of 0.268 in my rifle and conditions. The 168gr Barnes X has a G7 of 0.200. I get a better trajectory after 300 yards with the 200gr and within an inch up to that. The 2000 ft/s mark comes around 350 yards for the 168gr and 450 yards with the 200gr but the 200gr will performed as designed down to 1800 ft/s which puts it near 600 yards out.

For the two penetration is about the same as the 200gr will retain ~160 or so grains, but the destruction is much greater with the 200 NAB.

There is a lot more to it than SD alone.
 
Expansion characteristics affect penetration of course. X bullets are good penetrators because these have high weight retention and don't flatten out or fragment like some jacketed bullets do.
 
1) No, unless you consider that an expanded bullet also has a SD thet is determined by its expansion characteristics then yes.

2) Yes, and you'd better. Bear in mind that though the monos work better at higher speeds, the lighter bullets also slow down faster.
 
I believe so. For a good example, check out the ballistics of the .260 Remington (originally called the wildcat 6.5mm-08).

The BC of the .260 is pretty much ideal (about 0.4), making the .260 VERY popular with the long-range competition shooters.
And its combination of flat trajectory, high downrange energy, and low recoil is gaining fans among hunters as well.

However, the SD its 6.5mm bullet is so high that there are published warnings that it can penetrate overly deep
http://www.snipercentral.com/260-remington/
 
SD is irrelevant in 2017. Bullet construction trumps all, as long as impact speed is sufficient to allow the bullet to expand as designed. I would take a 30cal 130gr Barnes TTSX over a Berger 210gr VLD for hunting moose, elk, bear, at typical hunting distances of less than 400 yards. It's sectional density is garbage, but it will out penetrate the fragile missile
 
SD was reasonably important in the cup and core days. A heavy for caliber bullet, moving not so fast, penetrated well, whereas lighter faster bullets penetrated less. Which is why people still say "with a 30-06, you can use a 150gr for deer but need a 180gr for moose" :)


Right here on CGN, BC Steve has shot done lots of testing and you can compare SD to penetration.

I used to do lots of tests too, and one of them was this- all shot from a 300WSM

1930133_26043505515_8380_n.jpg



L-R

180 TSX 180 Failsafe 130 TTSX 180 NP 180 Hornady

They are in order of penetration, with the FS and 180 TSX being basically equal, the 130gr TTSX very close and then the 180 NP was a bit behind and the 180 Hornady far behind.

1930133_26043480515_4782_n.jpg


Which is why I don't worry too much about SD these days. If a 130gr TTSX will do the same thing as the time proven 180gr NP, why get worried about SD?
 
Sectional density, when referring to expanding bullets, was irrelevant 60 years ago and it is irrelevant now. It has validity when referring to solids but that is it. If you want to get technical i believe it is becoming more relevant with controlled expansion bullets as IMO it needs to be measured after expansion.
 
The only section density that matters, is the section density as the bullet actually passes through the animal. A bullet that starts out at 140gr and retains 95% of it's weight, can penetrate better than a bullet that starts out at 175gr, and sheds 40% of it's weight.
 
stubblejumper said:
The only section density that matters, is the section density as the bullet actually passes through the animal. A bullet that starts out at 140gr and retains 95% of it's weight, can penetrate better than a bullet that starts out at 175gr, and sheds 40% of it's weight.
Click to expand...

Yes and no. A bullets diameter is also important and that can be all over the map once there is impact.
 
stubblejumper said:
The only section density that matters, is the section density as the bullet actually passes through the animal. A bullet that starts out at 140gr and retains 95% of it's weight, can penetrate better than a bullet that starts out at 175gr, and sheds 40% of it's weight.
Click to expand...

In other words, bullet design and construction must be appropriate to the end use. Berger's idea of a game bullet is not mine, but I'll have a change of heart if I ever engage in long range game shooting, where in some cases, the bullet should also be annealed to ensure low velocity expansion. Conversely, I don't want to be the hunter who has a wounded bear/buffalo/lion at bad breath range, and a Berger in the chamber.

If extremely high velocities are your preference, you might find the rotational velocity too much for the construction of the bullet. If you are hunting heavy game, and have a shot at a lightly built animal, there might be insufficient target density in the lighter animal for the bullet to perform optimally, resulting in a slow kill.

SD is not a factor in any of these examples, yet I don't discount it completely. Chuck makes the point that SD is only relevant when bullets are designed to be non-expanding. I'll go a step further and say that SD can also be relevant when comparing expanding bullets of the same construction, that differ in caliber or weight. Consider a .308/180 TSX vs a .308/165 TSX vs a .277/130 TSX. A similar relationship would exist between Partitions, but SD is not a reliable point of comparison beteen TSXs and Partitions, since their terminal mechanics are so different.
 
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chuck nelson said:
Yes and no. A bullets diameter is also important and that can be all over the map once there is impact.
Click to expand...

And the SD of the bullet as it passes through an animal, is a function of how much the bullet expands. If the bullet expands to twice it's original diameter, the SD is reduced accordingly.
 
If one wishes to compare one Sierra bullet to another Sierra bullet SD has meaning, likewise a Speer HC to another Speer HC or a Nosler Part to another Nosler Part. The SD will tell you what to expect for penetration comparatively. As a general indicator amongst all the different construction of bullets it is now pretty meaningless. There are so many factors to terminal ballistics and so many bullet construction enhancements nowadays that the SD of a bullet plays a very small, almost negligible, role in penetration. The Nosler Partition was the very first improvement in bullet construction of expanding bullets, that started the negation of SD in gauging penetration.
 
stubblejumper said:
And the SD of the bullet as it passes through an animal, is a function of how much the bullet expands. If the bullet expands to twice it's original diameter, the SD is reduced accordingly.
Click to expand...

Right, sd will be reduced -

let w = bullet weight, assumed to be remain constant
let d = unexpanded bullet diameter
let d1 = expanded bullet diameter = 2d

sd = w/kd^2

sd1 = w/k(2d)^2 = w/k4d^2

sd1/sd = w/k4d^2 / (w/kd^2) = 0.25
 
c-fbmi said:
If one wishes to compare one Sierra bullet to another Sierra bullet SD has meaning, likewise a Speer HC to another Speer HC or a Nosler Part to another Nosler Part. The SD will tell you what to expect for penetration comparatively. As a general indicator amongst all the different construction of bullets it is now pretty meaningless. There are so many factors to terminal ballistics and so many bullet construction enhancements nowadays that the SD of a bullet plays a very small, almost negligible, role in penetration. The Nosler Partition was the very first improvement in bullet construction of expanding bullets, that started the negation of SD in gauging penetration.
Click to expand...

Does each Sierra bullet expand the same?
 
Slamfire said:
Right, sd will be reduced -

let w = bullet weight, assumed to be remain constant
let d = unexpanded bullet diameter
let d1 = expanded bullet diameter = 2d

sd = w/kd^2

sd1 = w/k(2d)^2 = w/k4d^2

sd1/sd = w/k4d^2 / (w/kd^2) = 0.25
Click to expand...

In many cases, the bullet weight will not be constant, and the bullet does not expand instantly, which makes a calculation, nothing more than an estimate. As well the bullet may expand and then pieces break off, so the remaining bullet is not always a good indication of the actual bullet diameter, or weight, as it passed through the animal.
 
chuck nelson said:
Does each Sierra bullet expand the same?
Click to expand...

If one is comparing Sierra Game Kings at the same impact velocity, then yes one could expect similar results from bullets with the same SD. In using SD as an indicator of penetration there were always a lot of assumptions that had to be made, but generally speaking a higher SD bullet of reasonable cup and core construction impacting within it's normal velocity parameters could be expected to penetrate deeper.
 
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