Bullet rotation vs velocity

[Buffalobob]

One of the more interesting features of killing animals is the energy of deformation. That is not normally a really big issue except it is important to consider when you shoot at marginal ranges as I often do. It is one of the reasons a hard cast bullet will penetrate so well. They have no energy losses from deformation so the momentum remains high. I never bother to calculate spin when I decide to calculate what the maximum range of my rifle is.

The question on the table is if you take a 45 caliber muzzle loader and shoot a 120 pound white tail deer with a round ball will the round ball expand more if it has a higher RPM when it hits the deer.

 

[davewilson]

can't say i've ever seen a round ball expand.they usually stay pretty round. maybe if they didn't use a 66 twist?

 

[Buffalobob]

can't say i've ever seen a round ball expand

It has no energy loss from deformation. The spin is immaterial other than for accuracy. All of its mass is retained and all of its momentum is retained.

Now then let us think about copper jacketed bullets. A person should remember that at 1000 yards a bullet is only going to rotate about twice as it goes through a deer and maybe three times in going through an elk. This thing is certainly no blender blade with big sharp blades of copper jacket spinning at high rotational speed.

Think about one more time. The bullet is going to spin around only twice as it goes through a deer. What kind of magic are people expecting to happen from that trivial amount of rotation. Go and get a bullet out of a box and a ruler and play with it a minute. Killing is from impact not rotation. Bullet expansion is going to happen (or not happen) from impact not rotation in the animal.

 

[RockyMtnMT]

I guess what I am thinking is that the rotation of the bullet is what is responsible for maintaining the path of the bullet on impact and after. Perhaps also keeping the bullet impact in line with the bullet in order for the bullet to perform as designed, concentrating the force of impact on the tip of the bullet in order to create uniform deformation. (That sound funny.) I had never thought of the bullet as blender blade, only the shape of the mushroom to create a wound channel by permanently displacing tissue.

What am I missing?

Steve

 

[Jon A]

What am I missing?

Nothing, I believe.

Bullet expansion isn't necessarily due to spin, but stability keeping it penetrating straight, point on, can allow it the opportunity to expand normally. If it turns sideways and tumbles before this has a chance to happen, obviously results will be very different.

This is very well documented for those of you who follow African DGR ballistic experiments where, especially with long monometal solids/semi-expanding bullets, twist has a HUGE effect on the outcome. Too slow a twist the bullet goes in, turns, tumbles and gives generally poor results. With a proper twist, the bullet stays straight and can even make a cup point solid expand a bit, which happens very slowly and gradually but won't happen at all if it tumbles first.

It has been a while since I followed the discussion closely, but at least a few years ago both Gerard of GS Custom bullets and Mike Brady of Northfork were very specific on the twist rates they stated their monometal solids required--not because they wouldn't stabilize flying through the air, but because the spin was required for them to stay stable and penetrate straight through the animal.

So yes, in my opinion, penetration tests with bullet RPM close to what it is when launched at max velocity from the muzzle, either by actually hitting the medium at long range or using a fast twist barrel at lower velocity will eliminate this as a possible source of error in testing.

 

[edge]

This link has some interesting thoughts on straight line penetration and why it happens/fails:

The Cure: Copper Huntbullets

edge.

 

[AJ Peacock]

This link has some interesting thoughts on straight line penetration and why it happens/fails:

The Cure: Copper Huntbullets

edge.

Nice article. It appears that the frontal area (upon impact and as the bullet's nose expands) has a lot to do with straight line penetration. How and if this affect is seen in "relatively" easy to penetrate animals like deer/elk (compared to Cape Buffalo, Elephant, Lion etc.) and the current Aluminum tipped bullets is still a question for me.


AJ

 

[davewilson]

BB, the spin has a lot to do with expansion of the bullet in the animal. it's the centrifugal force helping to bring a bullet apart after it's been compromized. it's why some bullets dust when they clear the barrel, because they can't take the centrifugal force. same thing happens when you hit something and the bullet starts to mushroom. once compromized, it expands much quicker. i've talked to several very experienced long range hunters and they go to faster than needed twist barrels for just this reason. they all said it was to get quicker, more reliable expansion.

 

[Buffalobob]

Dave
You start off where we agree, but then you go down the same path as many other people. A fast twist barrel certainly scores grooves in the bullet jacket and stresses the jacket more than a slower twist barrel. So when such a bullet impacts it may expand more. I said that in the first post. I have two rifles that are slightly faster twist than recommended for the bullets I shoot (one never knows before hand what a rifle will like so another forum member has some very nice, impossible to get anymore, bullets that my rifle didn't like even though it had enough twist).

After that part, you journey along with the others.

I'll remind you of one thing though, you had enough good sense to do your own tests and trust no one. A elk hunting trip for you or me cost $3,000 - $4,000. To have somebody else put up bad data, or make false claims or to just deliver a line of ******** and screw it up, is really irritating.

The guy asked a question that is very complicated and difficult. I showed him the fork in the road. He choose to go down the path that is well worn.

That fine with me. I am only responsible for my own self.

 

[edge]

On the original question of is more twist better?

IMO, you can induce problems by "over" stabilizing a bullet!

First, the faster you spin something the more imperfections will be exaggerated! For instance, no bullet jacket is perfect. The faster you spin the bullet the greater the dispersion that any imbalance will cause.


FROM LILJA: http://www.riflebarrels.com/articles/bullets_ballastics/bullet_imbalance_twist.htm

Second, think of a bullet as a gyroscope. If you try to alter its relationship with its direction of spin, it will resist. The same thing happens as gravity alters its course. The nose of the bullet will tend to yaw more the faster it spins.
A faster spin may help resist wind deflection on the plus side!


FROM: http://www.nennstiel-ruprecht.de/bullfly/index.htm

edge.

 

[panama]

as it was explained to me by Bo Clerke, my 22-243 mdlstd. with 1-9 twist pushing a 60 gr.horn. at 3960 was rotating at better than 312,000 he also pointed out to me when i showed him the target, the bullet holes had twin smudges in the direction of twist which he stated was jacket failer from velo.x rotation. i would think that any thing spinning at 312,000 rpm with 3000 fps impact velo. would cause massive tissue disruption whether it made one or two turns is irelevent at impact. just my opinon.