Calling all physicists - bullet stability

28 February 2002, 01:38

1894

I read something on the Sporting Rifles site the other day which made me wonder. The author thought that the long 6.5mm bullets would suffer more from touching twigs etc.

Given standard rifles and twists which bullet (assuming identical hardness and shape) is going to be more upset by an outside force?

243 9 twist 100gr bullet 2,900fps
6.5 8 twist 140gr bullet 2,650fps
308 12 twist 150gr bullet 2,800fps

And will a 6.5 156gr bullet at 2500fps be any more or less affected out of the same barrel?

Im interested in the physics not the folk lore.

28 February 2002, 08:34

<CARR4570>

I LOOKED THIS UP IN "UNDERSTANDING FIREARM BALLISTICS" BY ROBERT A. RINKER (A VERY BORING BOOK, BUT VERY INFORMATIVE) AND HE SAYS THAT GIVEN ALL THAT IS KNOWN THE BIGGER AND HEAVIER A BULLET IS THE BETTER YOUR CHANCES AND THAT MOST EXPERTS LEAN TOWARD MORE VELOCITY BEING BETTER. SO THE 308 WOULD FARE BETTER THAN THE 243. HE ALSO SAYS YOUR BEST BET IS TO TRY AND AVOID SHOOTING THROUGH BRUSH, AND HE'S NOT THE ONLY ONE WHOM I'VE READ WHO ADVOCATES THIS, IF AT ALL POSSIBLE

28 February 2002, 11:33

R-WEST

CARR4570 -

Why don'cha reach over with your left pinkie and hit the "Caps Lock" key? Caps make your posts hard to read, and, it's the Internet equivalent of SHOUTING.

BTW, I think the last part of your post is the most important - try to avoid the brush if at all possible.

R-WEST

28 February 2002, 12:04

Fritz Kraut

quote:
Originally posted by 1894:
And will a 6.5 156gr bullet at 2500fps be any more or less affected out of the same barrel?

It will work fine. Itï¿½s the most common bullet in this caliber here in Sweden and it stabilizes well.

Twigs and brushes are more or less fatal for any bullet if they are not hit close to the target. Stabilisation increases with the distance.

But, sorry, I donï¿½t have the proper physical explanations for it.

Best regards,

Fritz

28 February 2002, 12:50

<Don Martin29>

Here are the results of the testing I did by shooting thru hardwood brush into a very large target.

Every bullet that hit something keyholed.
Every bullet that hit something was deflected.
Some weak bullets (12 ga Foster slugs) and very high velocity varmint bullets blow up (.243 75 gr/ 3600 fps).
The more "powerful" the load the better it did. This may be due to kinetic energy tearing the branch away and momentum pushing it aside somewhat.

And as we all agree "don't hit the brush"

In order of sucess were the .375 H&H, 7mm Rem Mag, 30-06 and the .358 Winchester.

The .375 was quite a bit better than the others which were similar. As stated the .243 and 12 Forster slugs failed.

As to the merit or lack of of a long bullet I don't think there is much concern. They all tumble.

28 February 2002, 13:51

<CARR4570>

Sorry about the caps!

28 February 2002, 13:59

elmo

Robert Rinker (Understanding Firearm Ballistics) also states that given two bullets of the same weight and spin stability, the shorter fatter bullet will THEORETICALLY deflect less than a longer thin bullet due to the physical laws of gyroscopic precession. He then says that in the real world , that once ANY bullet hits brush it deflects badly and cannot be relied upon to do its job. I read and reread Rinker's book all the time. I think it's very facinating (Maybe I'm boring too!) and it's written in a way that non-rocket scientists can understand. It's made me a better rifleman, and it has paid for itself many times over in bar room bets!

28 February 2002, 14:37

<Harald>

Gerard hit me up for a better, physics based, optimum twist rate calculator than the old Greenhill formula. I am still wrestling with that because there is a lot going on and it involves pressures behind a shock and the yaw angle of the bullet.

In general, the longer a bullet, the closer it will be to a state of aerodynamic instability for any given velocity and twist rate. Lets ignore for the moment, the very real problem that exists for amplifying bullet wobble with fast twist rates due to eccentricities in the bullet and the issue of flying with an axial attitude highly divergent from the velocity vector (which is a pretty unlikely thing to occur, even at long range).

Longer bullets are heavier and that is a good thing, but here is the big problem. The bullet is a free body. Any free body acted on by an outside force (like air resistance or a tiny twig) WILL be moved. No matter how fast or heavy your bullet may be it will always be deflected by a twig. Whether it tumbles is a separate matter, but it will be diverted from its flight path even if it has the gyroscopic stability to "brush off the brush". It may not yaw much and still miss the target.

I have seen this dramatically demonstrated in real hunting situations in the last year and it is such an effect that I have no confidence whatsoever in any rifle's capability to bust brush. Shooting through a pine sapling as big as your arm is a whole `nother matter altogether - the bullet will be slowed, but if it goes straight through it won't be deflected nearly as badly as by a slender twig that hits it only on one side.

Rough guesstimate:

F = density / 2 x velocity^2 x surface area

F = 1000 kg / m3 / 2 x (800 m/s)^2 x (0.005 m)^2

F = 8000 kg-m/s2 <------------ a LOT!

F = ma = mass of bullet x acceleration

a = F/m = 8000 kg-m/s2 / 0.010 kg

a = 800000 m/s2 <---------------

t = s / v = 0.005 m / 800 m/s

t = 0.00000625 s

v = a t = 5 m/s <--------------- drift rate

TOF = 50 m / ~800 m/s = 0.0625

Drift = 0.0625 s * 5 m/s = .3125 m = 12.3 inches

28 February 2002, 16:55

denton

For spinning objects, there is a formula (long since forgotten!) that tells whether the spin is stable or unstable. It involves length, diameter, and some other variables.

Similarly, a weight hung from a string is in stable equilibrium. A pencil balanced on its point is in unstable equilibrium. Nudge the suspended wieght off center slightly, and it returns to equilibrium. Nudge the pencil, and it falls over.

Most bullets are calculated to be reasonably stable. The .223 is designed to be right on the brink of instability. As soon as it hits a target, it tumbles. This produces a much more serious wound, and was so successful that the Warsaw Pact nations protested loudly and vigorously that the M-16 violated agreements between them and NATO. They continued protesting right up until the time they adopted their own version of the same thing.

It's only a partial answer, but as much as I know about the question asked.