RPM/engraving vs. Terminal Ballistics

[sscoyote]

What's the collective opinion here? Would overstabilizing or overengraving a match bullets jacket effect its ability to come apart on impact, in effect creating a match bullet with varmint bullet characteristics??

 

[Michael Eichele]

Over stabilizing a bullet will surely make it tear apart more upon impact. The exception might be a solid copper bullet. The faster you spin a bullet, the more it will explode.

 

[MAX]

Over-stabilizing, I think not. Over-engraving, maybe...a little. All just my opinion but rpm contributes little to bullet expansion or tissue damage. Even very high twist rates will result in only 1-2 rotations in a deer on a broadside shot. The engraving thing, I'm not real sure about. Match jackets tend to be thin so if you have very deep grooves, perhaps this would contribute to frangibility, but it would be difficult to quantify. If I was going to worry about adverse effects from high twist rates my first choice would be inflight blow-up, the second, jacket slippage. Both would be evident on the target board.

Boyd Heaton has posted several photos of exit wounds from the SMK's. They make a big hole anyway.

 

[sscoyote]

Had a buddy of mine that was shooting a Savage .223 with a 1:9 twist, and I was vey impressed with the explosive effect his gun had on pr. dogs with the 50 Nosler. Most of the time it would blow parts of the dog to the right on impact. I now have an AR 1:9 twist, and was just theorizing if i could get better terminal ballistics at longer ranges from the VLD's with a 1:7 on a coyote with a deeper groove diameter. I think it'd be hard to quantify the difference, but theorizing's fun sometimes.

 

[Michael Eichele]

Allow me to rephrase, it is a scientifec fact that the faster the bullet spins, the more explosive it will be.

It is no coincidence that the 7mm Mag is so much more explosive than a 300 or 338. Unless the latter have very tight twists.

 

[sscoyote]

So theoretically assuming identical veocity the faster twist should be more explosive. I guess this is part of the problem with the .17 cal 37 gr. VLD. A lot of guys were experiencing bullet blowup on barrel exit.

 

[Tim Behle]

sscoyote,

Are you planning on building a new rifle to shoot those 37 grain pills?

 

[Guest]

The bullet looses rpm much slower than fps. At long range the bullet has lost a lot of forward speed but is spinning almost as fast as when it left the barrel. A 3000 fps 1/10 twist bullet will be a 1/5 twist down range when it drops to 1500 fps. I think that when a fragmenting bullet hits an animal the spinning of the bullet will send pieces of the bullet flying. A faster spinning bullet will make more internal mess than a slower spin.

 

[sscoyote]

No Tim-- i'm just trying to find a way to get better terminal ballistics from a lower veocity VLD bullet (AR-15). I'm not sure if there's ever been any research in this area.
I'm getting 3100 f.p.s. m.v. from a 70JLK VLD, and i'm trying to figure out what else, if anything, can be done to increase terminal ballistics, and at what range i need to draw the line for good terminal ballistcs.

 

[dwm]

You may want to consider that rotation contributes to the overal energy of a bullet. I hadn't really thought about that until reading this thread, but it is true.

So far every discussion on bullet energy has been focus on forward velocity, IE:

Energy (translational)= 1/2 * mass * velocity * velocity

Well rotation also provides enegry:

Energy (rotational) = 1/2 * I * w * w

where I = momentent of inertia and w = angular velocity

The total energy = E(translation) + E(rotation)

So a faster spinning bullet does indeed have more energy than a slower spinning bullet.

I am sure that when a bullet impacts and starts to deform that some of the rotational energy is converted to heat and contributes to the deformation and tissue damage.

Something to think about.

Interesting that I have never seen this in anything that I have read on ballistics and energy.

I will provide references if anyone dislikes the equations I provided above. It is basic Vector Mechanics.

 

[Michael Eichele]

dwm is right.

The rotation of a bullet also adds to the overall energy of the bullet, not just the forward motion.

On the other hand, you cant turn a 30-06 into a 300 ultra mag by turning the bullets faster.

Also, these exact topics and calculations are covered in "Understaning firearm ballistics" by Robert Rinker

[ 01-10-2004: Message edited by: meichele ]