Bullet Spin and Performance

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by buffalorancher, Dec 20, 2008.

  1. buffalorancher

    buffalorancher Writers Guild

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    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?
     
  2. Buffalobob

    Buffalobob Writers Guild

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    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/Products/OperationalMedicine/DATA/operationalmed/Manuals/NATOEWS/ch02/02Projectiles.html

    The Controversy about South African assault rifles

     

  3. buffalorancher

    buffalorancher Writers Guild

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    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.
     
  4. Michael Eichele

    Michael Eichele Well-Known Member

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    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.
     
    Last edited: Dec 21, 2008
  5. jwp475

    jwp475 Well-Known Member

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    How much cyntrifical force can the bullet have? What studies or proof shows a greater wound channel from over spinning a bullet?
     
  6. buffalorancher

    buffalorancher Writers Guild

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    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.
     
  7. BryanLitz

    BryanLitz <b>Official LRH Sponsor</b>

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    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
     
  8. royinidaho

    royinidaho Writers Guild

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    I tend to agree with Bryan based on my personal expansion tests.

    Testing at close range, with long range velocities, with several brands of bullets which opened properly and penetrated in a straight line distance into the media was as bullet perfromance was as expected.

    Bullets that were of insufficient velocity and/or suspected improper design immediately yawed upon media entrance, seemed to maintain that orientation through out their travel, did not travel in a straight line, and passed much further into the media. I can only suppose that the initial wound cavity with these bullets was much smaller than that of the bullets that performed properly.

    The next step in my expansion testing is to connect the RSI device to the media container to determine variations in "shock" or what ever that measurement reveals.

    What I have proven to myself is that SMKs need at least, if not a little more, than the 1800 FPS published FPS when launched at a 900 yd velocity. Maybe if launched at say 2700 FPS w/the media at 1K, the resulting increased RPM would make a bit of a difference on the bullet's ability more properly perform terminally in my media?
     
  9. Buffalobob

    Buffalobob Writers Guild

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    Bryan

    I ran some calculation of the angular velocity of the jacket of a 308 and while that number is bigger it was only as a prelude to an other calculation. Would you check to see if my calculations are correct. It has been many years since I had to calculate anything in radians.

    -----------------------------------------------------

    Muzzle velocity = 3000 ft/ sec

    Barrel 1-12 twist = 1 rev/ft = 1 radian

    3000 R/s w

    Linear

    2pi x r = v

    308 caliber r = 0.154 inches x 2 x 3.1416 x 3000/ sec / 12 inches = 241.9 fps




    Let us look at a 308 bullet from a 1-12 twist barrel fired at 3000 fps.

    It has a forward velocity of 3000 fps and an angular velocity of 3000 rev/ sec at the muzzle

    Converting the angular velocity to a linear velocity of a particle in the outer jacket can be done because we know the radius of a 308 bullet is half the diameter. That translates to 241.9 feet per second.

    If we spin the bullet with a 1-10 twist barrel then it has a linear velocity of 12/10 X 241.9 = 290 fps or a 20 percent increase.

    If we take the original barrel and decrease the muzzle velocity by 10 percent then the angular velocity is decreased by 10 percent.

    We can decrease the velocity by 20 percent and increase the twist by 20 percent and keep the angular velocity constant.

    If we go to a larger caliber bullet and keep the same barrel and muzzle velocity then the angular velocity will be the same but the linear velocity is more because the increase in the radius. So the difference between a 308 bullet and a 338 bullet is a radius of 0.154 inches versus a radius of 0.169 inches or about a 10 percent increase in radius and so a 10 percent increase in linear velocity.

    Nonetheless, no matter what twist nor what muzzle velocity nor what caliber, what I calculate is that the linear spin velocity is going to be about 10 percent of the forward velocity at the muzzle.

    The only reason angular velocity is important is to calculate angular momentum.


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  10. Michael Eichele

    Michael Eichele Well-Known Member

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    Studies done by myself and the testimonies of other experianced shooters. Some in ballistic gelitain, others on game itself.

    I challenge you to make some ballistic gelitain and fire a 190 SMK from a 12x barrel at a given velocity into it, then shoot a 190 SMK with a 9x barrel and see for yourself.
     
  11. RockyMtnMT

    RockyMtnMT Official LRH Sponsor

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    My understanding of the physics of twist on the bullet are not scientific. I have searched for information on this and then try to put it in practical terms that I can wrap my head around. So hear goes what I think I understand.:rolleyes:

    The stability factor of a bullet is dependant on the overall length of a bullet, the bearing surface of the bullet, the muzzle velocity of the bullet, and the twist of the barrel. What the bullet is made out of, and how the bullet is balanced will have some bearing on how much spin it will need, and or how much it can take.

    What I understand about over-stabilization is that too much spin can bring out the imperfection of a given bullet and cause it to go down range in a circular around it's flight path. Also it can cause the jacket to separate from the core partially or completely break the bullet into pieces.

    I understand that it takes less stability factor for shooting long range (beyond 500yrds) than for shooting less than 500yrds. This I believe has to do with the bullet nosing over and coming down with the point in line of the bullet path. If the stability factor is higher it will cause the point to stay in the upward position of the beginning of the flight and enter the target in an upward angle. I believe this can be very subtle and cause the bullet to not deform properly, and not stay on the intended line through the target.

    In the point of hunting we need the bullet to stay on the path that we aimed the bullet through the intended target. A bullet that may tumble will tend to pick it's own path after impact. This could be very bad on a quartering shot as you can imagine. As I understand it the higher rotation of the bullet forces it to stay on the aimed path. So one can conclude that as much spin as the bullet can physically take would be good for short range hunting (under 500yrds).

    In my quest for information on this subject I am beginning to think I need to have two loads for a given rifle depending on the distance of the shot. Perhaps this is hair splitting. I don't know. If the target is dangerous game it becomes more important for personal safety reasons.

    I also have concluded that the further we push these bullets (velocity and range) the more noticeable these factors become. The more radical the bullet designs get, for increased bc's, the more attention needs to be paid to the twist.

    This is a great thread, I hope we can get more involvement from the really smart people around here.:D

    Thanks for the time guys, Steve
     
  12. Lightvarmint

    Lightvarmint Guest

    Meichele

    10-4.

    And, if you cannot find some ballistic gelatin perform the following steps in order as an alternative:

    1) Go to your local range and dig out your favorite bullet that is mushroomed or has exposed cutting teeth on the circumference.

    2) Select your favorite power drill and chuck up the bullet and start it up at full speed. Bear in mind that it probably will only be 10% of the rated rpm of the bullet. The more rpm the more damage that will be done and the more rapidly it will be done.

    3) Next, either just think about what would happen if you grabbed the spinning bullet with your bare fingers. Now go find a piece of ham or roast and drill a hole in it with nothing but rotational force. I would not recommend grabbing it with your fingers, but it will provide a small visual example of the damage a rotating bullet can impart on flesh and meat when it is only at 10% of actual rotational velocity............ Just imagine the potential damage at full rpm and full range velocity.

    Finally, once you have done this, you will more fully understand the damage rotating mushroomed and/or fragmented bullets impart on targets and media. Now, you have to ask yourself will this be MORE or LESS significant damage than a broadhead launched at 300 fps from a comound bow at a range of 30-50 yards.....

    Also, if the bullet is stable upon impact and not in a yaw condition, it would tend to travel in the original direction upon impact. And, if you want to know what happens at 1000 yards, then test at 1000 yards to get the most accurate results.........

    Conducting long range testing at long range removes the doubt and forces us to discuss other topics.

    Lightvarmint
     
  13. Michael Eichele

    Michael Eichele Well-Known Member

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    That sums it up very very well. I dont always know how to translate my thoughts to a key board and you have done it to near perfection!

    Thank you Steve!

    Yes it may be hair splitting (having two loads for a given rifle depending on the distance) but that really would depend on how much of a perfectionist you are and what distances youre talking about. My opinion only here. I am a 1K shooter NOT a 1K+ shooter. I find that "the perfect" load works just fine from 0-1000 yards. More may be different.
     
  14. buffalorancher

    buffalorancher Writers Guild

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    This all good info. and I really appreciate it. How much would a bullet spin in an animal if you were using a 1:10 twist? I can see where spinning in a drill would illustrate some spin but if a bullet only turns maybe twice while in an animal this wouldn't be an accurate representation of what it does to an animal, would it? I envision this as a cork screw from hell instead of a spinning mushroom of death.