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> Bullet RPM and terminal performance.
Bullet RPM and terminal performance.
@ 11:23 am (GMT) |
Tom DixonSo we know that RPM affects stability but what does it do to terminal performance? Anyone have any observations?I was thinking that since bullets spin incredibly fast there must be a huge amount of stored energy just in the rotation irrespective of the speed. Varmint hunters choose fast spinning loads to get better terminal performance so how much difference does it make on game? If you imagine a bullet with no forward motion but spinning at 200,000+RPM it would be a tough thing to grab hold of. Anyone who has used a lathe would know the stored energy that a spinning object can hold. Also how quickly does the RPM decrease? is it at the same rate as the velocity? can we accurately calculate an impact RPM? RPM is calculated thus: MV X 720/Twist Rate (in inches) So the load that I posted about in the other thread which was a little too explosive for chest shots was spinning at 240,960 RPM if I had an 10 twist barrel it would have been 216,864 Rpm and if it had been an 8 twist it would have been as high as 271,080 RPM. I wonder as some of you have shot a lot of game with a lot of different rifles wether any noticeable effect has been seen with bullet performance and rpm. I know we choose twist rate with regards to bullet stability rather than performance but is there a performance difference? |
Replies
@ 03:26 pm (GMT) |
Bryan WebsterRe: Bullet RPM and terminal performance.I would not go to too tight a twist as it is normally not needed.Prior to buying a new barrel just decide what weight and style of bullet as well as range you will want to use and ask your barrel maker for his recommendation. This is all about bullet stability, not terminal ballistics. For more info I recommend you visit Berger bullet's web site and check out their info as well as Bryan Litz' books. If you have yet to buy Nathan's books, give them all a good read. |
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@ 05:49 pm (GMT) |
Tom DixonRe: Bullet RPM and terminal performance.I'm not looking for a new barrel Bryan, and yes if I was accuracy and stability would be the priority.I'm posting purely out of interest, does anyone notice a difference in bullet expansion, or meat damage relating to RPM. |
@ 08:02 pm (GMT) |
Mike DavisRe: Bullet RPM and terminal performance.its covered in the books...not just stability but how hard it is on projectile manafactures to make a one size fits all pillyou cant expect say a 130 grn pill made for a .30/30 going along at 2600fps to hold together if you poke it out at 3400fps out of a .30 magnum of some sort. out of even a humble .308 they become like a super varmit pill and make big messes thats an increase in velocity for sure but the increase in RPM just has to have a lot to do with it the same goes for Barnes type pills the faster they go the better they expand and if you look at a recovered projectile you will clearly see the effect of rotation as the petals look somewhat like a plane prop that hit the dirt. if you speed any pill up enough it will simple pull it self apart and wont even reach the target.(look at .220 swift in early days) Nathan talks about too fast of twist rate making a fine projectile into 2nd rate by pushing it past what its designer intended it for so they then have to make them harder/tougher and we get oposite effect where the plurry things are too hard for std chamberings and pencil through. again look at what is std twist rate in a .22/250 they are slow twist barrels as at the speed they going a fast twist is too hard on the soft varmit type projectiles the cartridge was designed for and thus the reason that guys who wish to use that round for deer a limited to the 45-50 grn pills normally as the barrels slower twist struggles with anything heavier. Vs the guys with fast twist .223 barrels on thier ARs so they can push 70 grn projectiles to stretch its legs a bit more... Id bet my dogs favourite bone those same guys will struggle with a varmit 50 grn pill unless they load it very mild. if you look at a sierra manual they have velocity recomendations for thier projectiles and have done so for years....... one day the rate of twist might have to be included too given how much guys are trying to push boundaries.... |
@ 11:19 pm (GMT) |
Warwick MarflittRe: Bullet RPM and terminal performance.Look at the length of your barrel and you see two and half turns of rifling over the 22 inches of its length. 217,080 rpm ÷ 60 seconds = 4,518 revolutions a second . In one second the bullet has travelled say 3000 feet and turned 4,518 times.........So 4,518 rpm ÷ 3000 ftps = 1.506 turns per foot of distance travelled. So if the bullet goes through a 4 foot thick target it will only turn 6.24 rotations if it hits no resistance to stop the torque (twisting energy ) the bullet has stored in it from the spin the rifling gave it. So if you look at it like above using the facts of the matter and what is really happening not what you think is going on! It would appear that the spinning is for stability and to attain accuracy? Look At a spinning top as it slows down it wobbles and goes off axis and back again until it's spinning is too slow to balance out the effects of gravity and it topples over. So it seamed the rpm was high except that the time scale was way too long for the flight time of the bullet. If you use feet per second you need to use revolutions per second to match.4,518÷3000=1.5 rotations per foot of distance travelled REMEMBER also that the speed of both are reducing as the time and distance increase! !! |
@ 11:59 pm (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Personally, I couldn't say what the effects would be. I was told by a fellow a long time ago that the rotational force is actually quite minimal. Speed does not necessarily make high torque. I'm sure there are some engineers who could give you the right numbers, but I think this fellow said something to the effect that it has more to do with the small amount of centrifugal (or he could have said centripetal) force generated by what we refer to as sectional density. His statement was hard to believe, but he said that if you could duplicate the rpm on a static object, you could stop the spin by merely pinching it between your fingers. |
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@ 12:16 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Tom, I think your analogy to the lathe is misleading, as the mass involved is probably in the tens of thousands of grains of weight compared to a bullets barely 200 grains.The difference in loss of rpm is probably quite minimal as, according to Newton, some other force has to act upon it in order for it to change. I'm guessing that the only measureable force acting on the bullets spin would be air friction/resistance, and with that small of a surface area, that force would in itself be small. I have no letters after my name, and barely made it through school, so don't believe anything I express. I have no proof. |
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@ 01:46 am (GMT) |
Mike DavisRe: Bullet RPM and terminal performance.if you have a .410 bail gun (without choke) and fired a .45 long colt round into pig at 5 yards then did same thin from .45lc rifle you could proove/disproove thissame goes with two rifles with different twist rates,why subject a projectile to faster spin than needed it just has to be hard on it. |
@ 05:47 am (GMT) |
Trace JacobyRe: Bullet RPM and terminal performance.Tom,I think the best way to answer your question is to think in terms of revolutions per distance traveled instead or revolutions per time elapsed. Putting bullet construction and stability aside, I believe your question is simply the effect of rotation on terminal performance, right? So a bullet leaving a barrel of, say, 1:12 makes one rotation per twelve inches of travel. Whether the bullet is traveling at 2400 fps or 3400 fps, this does not change. So when the bullet reaches the target, which could be 12 inches thick, it will make one revolution while traveling through the target (assuming that the target material or tissue offers no resistance to the revolution). By increasing the twist rate to 1:9 the bullet now makes one revolution per 9" of travel. That is a 25% increase, but still doesn't do much in terms of terminal performance when you think about such a small projectile covering a distance of 9-12" to make a complete revolution. This is not based on any experience with different twist rifles, but is purely hypothetical based on my understanding of how the bullets travel. I could be completely off base, hopefully someone else can correct me if that is the case. |
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@ 02:54 pm (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.The bullet with no forward momentum:Kinetic Energy = 0.5 X m X v(squared) m = weight of object in kg v = velocity of object in m/s KE = 0.5 X 0.0134kg X 41.0159m/s X 41.0159m/s KE = 11.2714 joules Bullet used in the above formula was a Hornady 208gr. 30 cal (.308"). Converting joules to inch pounds (using On-Line Conversion): 11.2714 joules = 99.760 inchpounds The kinetic energy in the spinning bullet is less than twice the rotational force you use to tighten your king screws on your pillar bedded action. |
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@ 03:54 pm (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Speed of rotation (RPM) when calculated from the formula for air resistance is so small ( 0.000 000 xxxN) as to be negligible. You can assume that RPM's remain constant for the flight of the bullet.http://www.softschools.com/formulas/physics/air_resistance_formula/85/ If you want to do your own calcs, fly at it. My brain hurts. |
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@ 11:51 pm (GMT) |
Warwick MarflittRe: Bullet RPM and terminal performance.I did the maths above just the basics. like I said you had to use RPS not RPM for the speed minutes and seconds don't give the proper answer |
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@ 12:02 am (GMT) |
Warwick MarflittRe: Bullet RPM and terminal performance.Here's a video of bullets in flight slow motion.: http://youtu.be/QfFoMyMoiX4 Watch "1 million frames per second Slow Motion video of bullet impacts…" on YouTube 1 million fps Slow Motion video of bullet impacts…: http://youtu.be/QfFoMyMoiX4 |
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@ 12:41 am (GMT) |
Mike DavisRe: Bullet RPM and terminal performance.which ever way you want to calculate it can we all agree that there is a significant amount of force being directed/pulled in an outward direction from centre of projectile??? ..... this MUST CONTRIBUTE to the way / speed in which a projectile opens/mushrooms upon impactso it then follows that if you increase the spin by either increasing the twist rate of barrel or speed of projectile the outward force must also increase.... if you go to the other extreme you could get yaw on impact and tumbling projectile...or even tumbling in flight. without spin or if spin isnt doing the above why wouldnt a projectile simply fold up or squash in on itself???? im yet to see a hollow point that has folded inwards instead of outwards yet it would be easier for it to do so as it is already going in that direction and there is more resistance from outside the cavity than from within it...........ive seen them folded over when impact speed has been low. but really doesnt matter just food for thought that makes my head hurt...lol |
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@ 01:13 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Interesting video. There are so many anomalies I don't think you could ever say that one size fits all. At the 1:02, 1:26, 2:10, 4:07, 4:20, and 5:46 it appears that the rotational momentum was arrested. Yet at 2:21 and 2:26 it appears that it was actually reversed. Say what????? Maybe it was the quality of recording. There is a lot of debris flying everywhere. It was also interesting that the different mediums behaved opposite to what you would think. Bullet construction is a major point. It looked like some Barnes' were fired through gelatin and the minimal mushrooming showed constant rotation. It was definitely an interesting watch, but there is so little analytical information to work with, not too many conclusions can be drawn. |
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@ 01:13 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Interesting video. There are so many anomalies I don't think you could ever say that one size fits all. At the 1:02, 1:26, 2:10, 4:07, 4:20, and 5:46 it appears that the rotational momentum was arrested. Yet at 2:21 and 2:26 it appears that it was actually reversed. Say what????? Maybe it was the quality of recording. There is a lot of debris flying everywhere. It was also interesting that the different mediums behaved opposite to what you would think. Bullet construction is a major point. It looked like some Barnes' were fired through gelatin and the minimal mushrooming showed constant rotation. It was definitely an interesting watch, but there is so little analytical information to work with, not too many conclusions can be drawn. |
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@ 01:29 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.I think you are correct, Mike. Once the bullet starts to mushroom, the material is probably in a very elastic/plastic/liquid state. While in that state, the centrifugal force would make the material flow outwards. But this would only apply in a medium that is resilient enough to let the bullet penetrate. In Warwick's video link, there were bullets that were fired against an unknown material that fully arrested the bullets forward momentum. If you watch the bullet open, the rifling scores do not show any rotational movement, yet they offer a weak point in the gilding metal and the force of expansion follows these and forces the bullet open into the petal shape. (7:33 - 8:00) |
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@ 04:30 am (GMT) |
Mike DavisRe: Bullet RPM and terminal performance.Paul,in my projectile box are 4 barnes TTSX projectiles that were fired at approx 3000 fps into a hunk of blue gum..end on so I could split log and recover them..... they went in about 4" which really impressed me as its plurry hard. all four look like adverts from Barnes,all are perfect little propellor shaped mushrooms, they clearly show rotation as petals are evenly peeled back and twisted around. also in that box is the single projectile we have recovered from game so far, into a red spiker at 250ish yards and that too is opening but not by very much,it still shows twisting happening,just like the adds say/show they do. |
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@ 04:53 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.There is little doubt in my mind that bullets do rotate inside the target. But how far into penetration are they still rotating? I don't suppose you still have that gum tree?This is also shown at those times in the vid mentioned above. All the projectiles are rotating, then comes the time they stop. It looks as if it happens as the mushroom reaches the bearing surface or close to it. Some of those bullets don't even mushroom, but this could be how or what they are made of. As I said earlier, not enough analytical info to form solid conclusions. But still, a very interesting topic. |
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@ 03:22 pm (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.It's funny what your mind does at 3:00 in the morning. One of Tom's original questions popped into the subconscious and there was a moment of mediocrity."I wonder as some of you have shot a lot of game with a lot of different rifles wether any noticeable effect has been seen with bullet performance and rpm." What came to mind was the fact that, in the evolution of firearms, previous to rifled barrels, lots of game was taken with round ball ammunition. There is no doubt that the accuracy and distances involved were considerably less than today's firearms, yet they still killed. A round lead ball with no rotation would still penetrate to the vitals. How much expansion/flattening is debatable. (Are there any primitive shooters here?). |
@ 05:40 am (GMT) |
Mike NeesonRe: Bullet RPM and terminal performance.We need to consider that energy is neither or gained if the same cartridge is used. If one was to use a slower twist, less energy would be required to spin the bullet and therefore more would be available for muzzle velocity... if a faster twist was used then more energy would be required to spin the bullet and less energy available for velocity. The amount of velocity change would be extremely difficult to measure as the energy required to spin is tiny when compared to the total energy stored in the cartridge. Either way energy will neither be lost or gained by changing twist. What twist can do, especially with frangible bullets, is to impart divergent tracks on the fragments to aid in wide wounding and also aid in bullets tumbling in regard to slower twist.In regards to a hollow point wanting to fold inwards... it won't fold in because the ogive supports itself much like the dome in the sistene chapel. The path of least resistance is to peel back. This was typed on my phone so apologies for any errors. |
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@ 12:14 am (GMT) |
Warwick MarflittRe: Bullet RPM and terminal performance.Its possibly possible to measure the amount of TorqueProduced by the charge of powder if you mounted the action into a bed with load cells To measure rearward forces of recoil and rotational torque forces. The answer is available with some intelligent testing. I guess you'd call it a rifle dynommeter as you're doing the same thing as race car teams do to test vehicle horsepower and torque performance.! Given a few zillion $$$$$$ I'd buy some load cells and build a rifle test bench that tells you everything. .... It's not hard to be intelligent..... but its easier being dumb...... |
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@ 12:58 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Way back last century when I was just a little tyke, there were some experimenters tied in with the US army or government. What they were doing was testing ultra-high velocities in the neighbourhood of 5000fps. What they had found, according to one article, was that the energy needed to impart twist/rotation could be as high as 35% of the full charge. Now, it didn't say that 35% was always used, what they said was it was "up to". No further information was available (govt.) except that they ended up using smooth bore barrels. |
@ 01:23 am (GMT) |
Paul CartyRe: Bullet RPM and terminal performance.I've been reading this forum for a while, but yet to post anything... Another factor with increasing twist rates is losses in the form of friction. As the twist rate increases so too does the angle of attack on the projectile, causing a consequent increase in friction. So some of the potential energy of the charge would go towards spinning the projectile but there's also losses involved. I'm not sure how much but it could be quiet significant and may give us a clue as to the effectiveness of friction coatings like Lubalox etc. |
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@ 04:20 pm (GMT) |
Bryan WebsterRe: Bullet RPM and terminal performance.The need for faster barrel twists is not dependent on wanting to improve terminal ballistics. We all have heard mention of light bullets spinning too fast causing blowup in the air and maybe even on impact and do not need that. The internet is full of mention of new rifle builds with fast twists etc for a lot of reasons, perhaps some right and some wrong ones.In my opinion all that matters is bullet stability. If I am shooting a 7mm Remington Magnum with a 162 grain bullet in a 1:9.5 twist barrel I am going to be a happy camper - until Hornady announces they are going to release a new ELD-X bullet in 180 grain that is gong to be really long. A 1:9.5 twist may not stabilize it, so as my current barrel is shot out I ordered a Benchmark 1:9 twist barrel and the barrel maker says it will be perfect for that bullet...just waiting now. We hear all the time about someone buying a .243 or .223 then realizing he or she cannot shoot the heavier more recent bullets and those who give these people advice online tell them they need a faster twist barrel. Yes they do, but do they always really need to shoot those heavier bullets that do really well in their present rifle/barrels. OFten it is not the case but more the internet hype related to long range competition shooting that has created an interest in doing these longer kinds of ranges. More power to them, and as we are finding an ever increasing interest in longer range shots, there is also a lot more interest by a lot more people in getting into shooting and hunting sports. I understand what this thread has leaned into as far as bullet RPM, but when I switch to a 1:9 twist in my 7mm Remington Magnum, I doubt that it will ever be used with the lighter bullets I often used in it with the 1:9.5 although there is little doubt they would be fine if perhaps at a bit less velocity. I recently bought a box of Sierra heavy 7mm matchings that say they must be shot from an even faster twist bore and I will likely try them on targets before banging steel in our part of the world in open country with ever present high winds. Certainly the velocity is not going to be high but they will be stable and defy wind better. |
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@ 04:20 pm (GMT) |
Bryan WebsterRe: Bullet RPM and terminal performance.The need for faster barrel twists is not dependent on wanting to improve terminal ballistics. We all have heard mention of light bullets spinning too fast causing blowup in the air and maybe even on impact and do not need that. The internet is full of mention of new rifle builds with fast twists etc for a lot of reasons, perhaps some right and some wrong ones.In my opinion all that matters is bullet stability. If I am shooting a 7mm Remington Magnum with a 162 grain bullet in a 1:9.5 twist barrel I am going to be a happy camper - until Hornady announces they are going to release a new ELD-X bullet in 180 grain that is gong to be really long. A 1:9.5 twist may not stabilize it, so as my current barrel is shot out I ordered a Benchmark 1:9 twist barrel and the barrel maker says it will be perfect for that bullet...just waiting now. We hear all the time about someone buying a .243 or .223 then realizing he or she cannot shoot the heavier more recent bullets and those who give these people advice online tell them they need a faster twist barrel. Yes they do, but do they always really need to shoot those heavier bullets that do really well in their present rifle/barrels. OFten it is not the case but more the internet hype related to long range competition shooting that has created an interest in doing these longer kinds of ranges. More power to them, and as we are finding an ever increasing interest in longer range shots, there is also a lot more interest by a lot more people in getting into shooting and hunting sports. I understand what this thread has leaned into as far as bullet RPM, but when I switch to a 1:9 twist in my 7mm Remington Magnum, I doubt that it will ever be used with the lighter bullets I often used in it with the 1:9.5 although there is little doubt they would be fine if perhaps at a bit less velocity. I recently bought a box of Sierra heavy 7mm matchings that say they must be shot from an even faster twist bore and I will likely try them on targets before banging steel in our part of the world in open country with ever present high winds. Certainly the velocity is not going to be high but they will be stable and defy wind better. |
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