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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? |
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@ 10:22 pm (GMT) |
Tom DixonRe: Bullet RPM and terminal performance.I get the argument relating to number of revolutions per foot travelled and that this will be not very much in the body of an animal. However we need to remember that the bullet will slow down dramatically on impact. Probably better to think of revolutions per second and then consider the time the bullet spends inside the body of the animal.So my load at 240960 RPM is doing 4016 RPS, lets say it spends 0.1 seconds inside the body of an animal thats still 400 revolutions... |
@ 04:16 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Even if your bullet had an impact velocity of 1400fps, in 0.1 seconds, it would travel 140 feet. That's a big animal. So let's take it down to 0.001 seconds. That would take it through 1.4 feet, or 15". More realistic. So the number of revolutions would be four.(I sure hope my arithmetic is right. My apologies if it isn't, it's been a long day.) |
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@ 04:18 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Sorry, 17", not 14". Told you it was a long day. |
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@ 04:18 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Sorry, 17", not 14". Told you it was a long day. |
@ 06:36 am (GMT) |
Warwick MarflittRe: Bullet RPM and terminal performance.With no wind resistance. At 3000 ftps. In one minute the bullet would travel 54,864 meters that's 54.8 kilometers! !! I'm going to stick my neck out of the trench and state that bullet rpm is solely for stability and accuracy. That a hollow point bullet gets stuffed with fur and skin and that it hydraulics inside the hollow point and acts like a wood splitting wedge forcing the thin copper tip of the bullets hollow point to roll outwards and increase the frontal area catching fat and muscle and getting wider until the force of the bullet becomes equal to the resistance of the animal and then lessens until the bullet exits the other side because its run out of animal to bore through or runs out of energy and stops..... sit on your cat and see what happens? Newtons law is why it happens. "For every action there's an equal and opposite reaction".Stop talking and get out shooting. ! |
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@ 11:51 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Yeah, get out shooting.......easy for you to say. What is it down there? 20*? 25*? Come on up here and try it. I just went outside to warm up the car and it's -32*. You go outside and shoot. |
@ 05:07 pm (GMT) |
Bryan WebsterRe: Bullet RPM and terminal performance.Was out this morning to sight in a 300 Weatherby and a .308 Win. Was -28 and it did not take us long at all. One group to start was 3 shots from each rifle at 100 yards, moved the turrets to the appropriate point and fired a 5 shotgroup from each rifle at 200 yards. Done. The .308 group was 0.4 on centres and the Weatherby was 0.5 on centres. Nice to get back in the truck. Now to kill more wolves and a few coyotes. |
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@ 05:25 pm (GMT) |
Tom DixonRe: Bullet RPM and terminal performance.Quote: Even if your bullet had an impact velocity of 1400fps, in 0.1 seconds, it would travel 140 feet. That's a big animal. So let's take it down to 0.001 seconds. That would take it through 1.4 feet, or 15". More realistic. So the number of revolutions would be four.
(I sure hope my arithmetic is right. My apologies if it isn't, it's been a long day.) Yes but your assuming that the velocity doesn't change inside the target, whereas it will slow down considerably. That's my point, it may impact at 1400fps but it certainly won't be going that fast on exit, the time spent inside the beast will therefore be much longer.. |
@ 07:58 pm (GMT) |
mark korteRe: Bullet RPM and terminal performance."Yeah, get out shooting.......easy for you to say. What is it down there? 20*? 25*? Come on up here and try it. I just went outside to warm up the car and it's -32*. You go outside and shoot."Easy Paul, easy - it keeps the riffraff out, it keeps the riffraff out...... |
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@ 08:45 pm (GMT) |
Warwick MarflittRe: Bullet RPM and terminal performance.URL=http://s1143.photobucket.com/user/Its not easy Paul! No bears wolves or fox's here. No cat to sit on either! I have worked in blizzard conditions and in underground mines 52 deg Celsius hot!!! When it's hot you can only take off some clothes and its still hot! In the cold you can put on more clothes and keep warm. I know that its not easy though. Cabin fever sucks! I bet. |
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@ 08:49 pm (GMT) |
Warwick MarflittRe: Bullet RPM and terminal performance.
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@ 12:53 am (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Nice digs, Warwick. Our skies have been like that for a while now too. That is the bonus of the cold.....nice blue sky and brilliant sunshine. It makes up for the temps. (Trees are a slightly different colour, though.) Thanks for sharing and reminding me that it will be here soon enough. |
@ 06:40 am (GMT) |
Mike DavisRe: Bullet RPM and terminal performance.here is my buddies take on thisimjim Super Member Joined: May 17, 2009 Posts: 6699 Location: Fort Worth TX Posted: Wed Dec 21, 2016 2:03 am Post subject: Re: twist rates and end results First, it is centrifugal force that can cause a lead-core bullet to disintegrate in flight. It’s not barrel twist and it is not velocity, it is both combined – twist rate x velocity = RPMs. A barrel with a slower twist rate can spin a bullet at a higher RPM if shot at a higher velocity. The first indication I found that bullet spin rate affected bullet expansion was with the Barnes .277 110 TTSX. A hunter was claiming great results in the 6.8mm SPC and showed a beautifully expanded bullet he had recovered while the expansion I was getting was dismal. The only difference was spin rate. I confirmed this with a terminal performance test shooting the 110 TTSX from a 1:11.25 and 1:10 twist at the same velocity. The .270 Win is typically a 1:10 twist and the 6.8 is 1:11.25 plus the .270 launches the bullet at a higher velocity (3400 vs 2700 fps) resulting in a spin rate of 244,800 RPM vs 172,800 RPM, respectively. The added centrifugal force helps the TTSX bullet expand to larger diameters. Based on this finding, I did some dedicated testing with .277 and .223 bullets. I did not find a difference in expansion vs bullet RPM with the Barnes TSX (non-tipped) bullets. I've tested the TSX and TTSX across the caliber spectrum from .223 to .308 and the TSX has always out-expanded the same weight TTSX in the same caliber (does not apply to Barnes new LRX). This is probably why Remington chose the TSX for their Hog Hammer ammo line. _________________ "To anger a conservative, lie to him. To anger a liberal, tell him the truth." - Theodore Roosevelt "The world is a dangerous place to live; not because of the people who are evil, but because of the people who don't do anything about it." - Albert Einstein Back to top slimjim Super Member Joined: May 17, 2009 Posts: 6699 Location: Fort Worth TX Posted: Wed Dec 21, 2016 2:04 am Post subject: Re: twist rates and end results So, right off the bat, I found bullet spin doesn't affect the expansion of every bullet the same. The .223 has even a wider range of twist rates so I did some focused testing there. . In the .223 where twist rates can vary from 1:7 to 1:11 or slower, I found the RPM effect on bullet expansion can be dramatic and not measurable. In the case of Hornady's new 55gr .223 GMX bullets, it will expand as much as 25% more when the bullet is shot from a 1:8 or faster compared to a 1:11 (see graph below). You can also see that the 55gr GMX bullet became elastic just before it lost its pedals around 1100 ft-lbs of energy resulting in some amazing expansion with the faster twist rates. I also tested some Nosler 64gr Bonded and Federal 62gr Fusion bullets. These are bonded lead core bullets and there expansion was not affected by twist rate – each bullet looked like a clone of the one before. 02 hornady 55 gmx expansion with twist rate.jpg Description: Filesize: 52.48 KB Viewed: 30 Time(s) _________________ "To anger a conservative, lie to him. To anger a liberal, tell him the truth." - Theodore Roosevelt "The world is a dangerous place to live; not because of the people who are evil, but because of the people who don't do anything about it." - Albert Einstein Last edited by slimjim on Wed Dec 21, 2016 2:12 am; edited 1 time in total Back to top slimjim Super Member Joined: May 17, 2009 Posts: 6699 Location: Fort Worth TX Posted: Wed Dec 21, 2016 2:05 am Post subject: Re: twist rates and end results Also, note barrel twist rate only stabilizes bullets enough for flying through the air, not during the terminal phase penetrating through flesh. For a bullet to remain stable and penetrate on-course after impact, the bullet has to shorten and increase/expand in diameter on contact. If not, the bullet will immediately tumble and 1) break in the middle (fragment), e.g., 110 BTHP, 115 Nosler CC, and other OTM bullets ... or ... 2) yaw 180 degrees and penetrate tail first, e.g., .277 130gr VLDs. Even if the bullet expands and shortens it might not be enough to remain stable during penetration. If you take a recovered bullet, measure its length and diameter, and run it through a stability calculator using velocity at impact. The stability factor is around 40 after impact vs 1.5 for stability in-flight. More centrifugal force (faster twist rate) can increase that diameter of expansion for some bullets which should result in a higher rate of energy transfer and likely reduce penetration. _________________ "To anger a conservative, lie to him. To anger a liberal, tell him the truth." - Theodore Roosevelt "The world is a dangerous place to live; not because of the people who are evil, but because of the people who don't do anything about it." - Albert Einstein |
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@ 06:41 am (GMT) |
Mike DavisRe: Bullet RPM and terminal performance.didnt get his graph in middle bit but can get gist of it from text. |
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@ 12:06 pm (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Good morning, Mike. Is there anyway we can get the original test/graph results your buddy came up with? I'm not sure what Nathan's policy is about putting another website link on his site, so maybe ask him first.His findings could prove to be conclusive, but the way they are represented in his posts are vague and very generalized. There are also statements that seem to contradict his findings (this may just be my interpretation and this is why the graphs/results would be helpful). Without any backup info on methods and analysis, it's hard to draw any conclusions. It's probably just me being cynical and anal. |
@ 09:03 pm (GMT) |
Nathan FosterRe: Bullet RPM and terminal performance.Right, must be my turn.When the 1:8 twist 7mm first arrived on mass, one of our NZ chop shops tried to sell me one of these on the basis that the fast twist caused increased wounding. Fast forwards a few years and the whole scene had changed. The fast twist barrels were now causing problems with jacket slip, forcing Berger to adopt a heavier jacket to prevent this problem. The net result was a decrease in terminal performance due to extra tough jackets. So here I am a few years on, studying a nice Kreiger 8 twist barrel on a custom rifle which puts 7mm bullets sideways at 100 yards. The client does not know who to blame, the smith he originally went to is equally confused. The bore is dimensionally sound- Krieger did their very best. Its a dud, it cannot even handle the post 2011 180gr VLD, take it off, throw it away. The extreme forces of a fast twist and high velocity simply increase bullet heat. When it comes to terminal performance, at least at close to moderate ranges, we have to ask ourselves, are we talking about RPM or temperature? Having been on this merry go round before, again and again, I will say as other already have, that flight stability is the key factor that we should focus on, not terminal performance. With regards to "more" expansion of the Barnes. The Barnes can only expand so far via its design, after which the petals fold back against the shank with a decrease in frontal area. The faster it is driven into something, the more the petals fold back. Some centrifugal force may help keep the petals out wide but this does not change the general nature of this bullet when used across the velocity spectrum, not just at one impact velocity. At lower impact velocities, the Barnes can still expand and the frontal area may be wider than at high velocity. Yet- the wound is smaller as a result of decreased hydraulic forces which has little to do with twist. Try drilling a Barnes wide and down deep to see what happens- nothing. The design is what it is. If you want a significant change, use another bullet design. I have seen a lack of twist cause tumbling on impact, but cannot state that this is a bad thing because at these very low velocities, no copper clad bullets can expand and therefore tumbling as a form of energy transfer is highly desirable. It was also very useful in .223 with FMJ pills. The one thing I have noticed more than anything, is fast twist rates ruining project rifles. Guys use the Berger calculator, opt for a super fast twist, then add so much velocity (magnum power) that the rifles become extremely finicky. Berger say its better to have more. I say its better to have a bit less. Most target folk shoot / test mild cartridges, I shoot everything and with varying measures of freebore and with the goal of target rifle accuracy from as an example, sporting magnums- and I think thats the difference. At the end of the day, bullet construction is the key. The distance you shoot to will also make a difference. If you shoot a magnum with a fast twist at close ranges, bullet impact temperatures can be slightly higher than a slow twist. That's why the wound looks grey if you have ever witnessed this, the bullet was molten on impact. One could argue this back and forth for days (as seems to have happened here). The temperature of the bullet, the stability on impact, not just flight stability, the rotational forces. But always remember the importance of bullet construction. Hence why the quoted post was without a conclusion, bullet construction had the greatest impact on results. Whatever you do, do not let some smooth talking salesman bullshit you into thinking that an extra fast twist is the key to accuracy and fast killing. One thing is for certain, I have never seen a .308 shooter shift from 12 to 10 twist and state that it kills a whole bunch better at long ranges. |
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@ 07:22 am (GMT) |
Mike DavisRe: Bullet RPM and terminal performance.Paul sorry im a bit of a techtard and fail miserably trying to post links on here.www.huntingnut is the site where the above was taken from.....I (Elvis) asked the question as some very knowledgeable chaps on there. |
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@ 02:21 pm (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Thanks, Mike. I'll give it a read. |
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@ 03:37 pm (GMT) |
Paul LevermanRe: Bullet RPM and terminal performance.Having read through the above quoted exchange on another site, I would first like to say that my comments that follow are not argumentative, strictly my own personal opinion.I do not doubt that the author's findings are accurate in regards to the actual physical measurement of bullet expansion. What does make me question his results is the lack of information in his reporting of his findings. The author also makes very generalized statements that have very little to do with the testing and are made in such a way as to make the reader believe that the statement is fact. It also appears that his methodology is flawed, which in turn, will influence the conclusions he came to. Until more information is made available, I would be hesitant to accept his findings. |
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