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Bullet Spin and Performance

 
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Old 12-20-2008, 09:59 PM
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Bullet Spin and Performance

There has been some discussion in other threads about bullet spin and the resulting performance or lack of. We all know which threads I'm talking about. It is impossible for me to comprehend the rotational velocity of a bullet but I have a hard time understanding how this rotation, or I should say the change of, can significantly change bullet performance. For the sake of argument we use a 10 twist barrel so, regardless of velocity, wouldn't the bullet only spin roughly 2 times if the chest of the animal is 20 inches wide? Or does the velocity shed that much faster than the RPM for this to be significant? The bullet still wouldn't spin that many times while in the animal if there is a pass through with much retained velocity.

When I started shooting long range a couple of years ago it was obvious right away that bullet performance at low velocity and the resulting poor peformance was something I had never encountered. Over the last 15 years we have killed literally hundreds of buffalo with just about every bullet available between 30 cal. and 45 cal. and I've never seen what I would consider a catastrophic bullet failure unless something was hit before impact. These animals were all taken with many different RPM's but never further than probably 300 yards. I question if this is really as significant as it has been made out to be. Could somebody else shed a little light on this?
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Old 12-20-2008, 10:43 PM
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Re: Bullet Spin and Performance

Here is some light reading for you. Whether these people are experts is something you have to determine yourself. There is a lot of stupid stuff on the internet.

http://www.brooksidepress.org/Produc...ojectiles.html

The Controversy about South African assault rifles

Quote:
The slightly longer and heavier American M855 bullet shot from the M16A2 assault rifle is replacing the M193 bullet shot from the M16A1 as the standard bullet of the US armed forces.

FN Herstal originally developed this bullet type (which has a steel "penetrator" as the forward part of its core) designating its bullet the SS109. The wound profile is very similar to that produced by the M 1 93 bullet.

Although the SS109 and M855 are not the same bullet, their differences are small and one almost needs a magnifying glass and a side-by-side comparison to differentiate the two. There is little difference in their performance in tissue.

The abdominal and thigh wound produced by the M855 or the SS109 bullets would essentially be the same as those described above for the M16A1 M193 bullet.

The longer 5,56mm bullets (M855, SS109) need a higher relational velocity to maintain stabilization in air. FN claimed that this faster rotation also causes the SS109 to have a significantly longer path in tissue before marked yaw occurs, thus producing wounds of less severity. This is simply untrue (compare Fig 3 with Fig 4). Additional rotation beyond that needed to keep the bullet straight in air appears to have little or no effect on the projectile's behaviour in tissue.

However, there is a situation concerning rotation rates whereby these longer 5,56mm bullets can cause increased wound severity. Shooting the SS109 or M855 in the older M16A1 rifle (they are not intended for use in this 1-in-12in. twist barrel, but iii the newer l-in-7in, twist) produces a bullet spin rate insufficient to stabilise the longer bullets. Such a bullet will yaw up to 70 degrees in its path through air. Striking at this high yaw angle (essentially travelling sideways), these bullets break on contact and the marked fragmentation, acting in synergy with the temporary cavity stretch, causes a large (over 15cm) stellate wound with the loss of considerable tissue.
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Old 12-20-2008, 11:17 PM
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Re: Bullet Spin and Performance

Buffalobob,
It appears that these people are saying that an understabilized bullet will hit the target sidways a inflict more damage. I can buy that, but they are also saying that any additional RPM after stabilization doesn't make any difference which is contradictory to what has been said here. For some reason this makes more sense to me.
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Old 12-21-2008, 02:39 AM
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Re: Bullet Spin and Performance

Quote:
Originally Posted by buffalorancher View Post
Buffalobob,
It appears that these people are saying that an understabilized bullet will hit the target sidways a inflict more damage. I can buy that, but they are also saying that any additional RPM after stabilization doesn't make any difference which is contradictory to what has been said here. For some reason this makes more sense to me.
There are 2 parts to over spinning a bullet. One part is the affect on game. The other is the affects in exterior ballistics. Exterior ballistics being what happens between the muzzle and the game.

In regards to killing power, additional RPMs (greater than what is needed for PROPER stabilization) will cause several things. A greater wound chanel in game due to cyntrifical force. It creates more temporary cavitation inside the critter by way of a shock wave. this is caused by the high RPM's. The higher the RPM the greater this "shock wave". Some bullets will also shed more jacket material into the animal due to this higher RPM and subsequent cyntrifical force.

From an exterior in flight ballistics standpoint it leads to lower BCs from causing the BC to decay more rapidly and more inches of spin drift downrange. Bullets that are stabilized to a 1.5 stability factor will have the best BC potential and the least amount of spin drift. It is better to have a bullet slightly over stable than under stable but the best is 1.5 The reason we have stability factors is because it obviously takes more or less twist to stabilize a given bullet. It may take a 10 twist to stabilize a 240 SMK yet only a 12 twist for a 190. Both will have close to 1.5 stability factors here. This is where they will be at their optimum performance in the air. As far as exterior ballistics are concerned, the closer you can put a bullet to sleep to the end of your barrel the better off you will be. Note that stable and sleep are 2 totally different subjects.

To explain sleep better, all bullets have a prefered or better yet "optimum" RPM in which they will settle down and have the most perfect stabilization. This is why some rifle/bullet cominations have larger MOA patterns at closer ranges than longer ranges. In some dramatic cases, groups will be smaller at long range and larger at short range. A bullet may be over spun to a point and as the bullet decelerates finds its optimum RPM and goes to sleep. Once asleep, they typically wont wake up untill they hit the transonic wall. Some may continue on and maintain somewhat stable and others will loose everything and keyhole untill the tumble.
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Last edited by Michael Eichele; 12-21-2008 at 02:54 AM.
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Old 12-21-2008, 06:18 AM
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Re: Bullet Spin and Performance

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Originally Posted by meichele View Post
There are 2 parts to over spinning a bullet. One part is the affect on game. The other is the affects in exterior ballistics. Exterior ballistics being what happens between the muzzle and the game.

In regards to killing power, additional RPMs (greater than what is needed for PROPER stabilization) will cause several things. A greater wound chanel in game due to cyntrifical force. It creates more temporary cavitation inside the critter by way of a shock wave. this is caused by the high RPM's. The higher the RPM the greater this "shock wave". Some bullets will also shed more jacket material into the animal due to this higher RPM and subsequent cyntrifical force.

How much cyntrifical force can the bullet have? What studies or proof shows a greater wound channel from over spinning a bullet?
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Old 12-21-2008, 09:12 AM
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Re: Bullet Spin and Performance

I can believe all the theoreticle concepts with regards to bullet spin but, from a functional standpoint, I can't see anything other than a huge increase or decrease having any practical effects. The range of velocities and barrel twists we generally have in ALL firearms used for sporting purposes isn't really that huge and most hunting bullets will perfrom at least reasonably well with in the relm of most modern day firearms. A bullet that is used for hunting and needs a specific RPM to perform correctly is not going to be a very good hunting bullet.
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Old 12-21-2008, 10:22 AM
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Re: Bullet Spin and Performance

I'd like to point out some calculations regarding translational and rotational kinetic energy that are completely open for interpretation.

barrel/bullet:
A typical .30 caliber 155 grain bullet at a muzzle velocity of 3000 fps from a 1:13" twist barrel.

At the muzzle, this bullet will have 3095 ft-lb of translational kinetic energy.
At the muzzle, this bullet will have 7.7 ft-lb of rotational kinetic energy.

When the bullet has slowed to 1/2 it's original muzzle velocity, it will have 1/4 of it's original translational kinetic energy. This will happen somewhere around ~800 yards. At that distance, the rotational speed of the bullet will have decayed as well, but not as much as the forward velocity. The rotational velocity at this point will be about 80% of it's original value. So the numbers after the bullet has slowed to 1500 fps are:

Near 800 yards, the bullet will have 774 ft-lb of translational kinetic energy remaining.
Near 800 yards, the bullet will have 4.9 ft-lb of rotational kinetic energy remaining.

As I said, these numbers are open for interpretation. I can tell you what I think about them, but it's just speculation.

At the muzzle, the rotational KE is only 0.2% of the translational KE. This tell me that practically all of the damage done will be due to the bullet's forward motion, the spin being of little consequence to terminal performance.

At around 800 yards, the rotational KE has grown to 0.6% of the translational KE. This is 3X higher % than at the muzzle, but still an insignificant amount compared to the translational KE.

I am of the opinion that the rotational speed, and rotational KE of a bullet are of very little issue to that bullets ability to do damage on a target. I could be wrong (I've never done the test), but the numbers suggest that there is very little energy in rotation compared to forward motion.

-Bryan
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