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> Twist rate and HUNTING bullet RPM speed limit?
Twist rate and HUNTING bullet RPM speed limit?
@ 07:50 am (GMT) |
Howard AudsleyI have a 6mm Creedmoor which has a relatively fast 7.5 twist barrel. I have heard that there is a point which if revved up too fast, assuming a hunting bullet survives the trip may still explode on impact due to centrifugal force. The concern is if a bullet used for hunting game just explodes on the surface and does not survive to penetrate.If so, what is this RPM speed limit? I have heard the number 300,000 RPM at muzzle, which this 7.5 twist 6 CM is more than capable of doing. Thanks! Howard |
Replies
@ 07:40 am (GMT) |
Nathan FosterRe: Twist rate and HUNTING bullet RPM speed limit?Hi Howard, I will quote Hornady ballistician Dave Emary's answer for all ballistics phenomenon by saying - 'it depends'.It starts within the barrel. If the contact area is very high (highly finished bore), the heat generated by both the contact area and extra fast twist can heat up a thin jacket bullet to the point that it loses integrity. Conversely, a rough bore, even if it produces a relatively high degree of copper fouling, may not generate the same level of friction. I say this based on first hand experience regarding efforts to determine cause and effect. Jacket thickness is also an obvious factor. A thick jacket bullet handles heat better than a thin jacketed item. That said, this is a bit of a moot point when discussing 6mm as most feature very thin jackets. You can employ a mono, but in the sub cals, monos can produce terrible results (slow kills) once the initial velocity falls away. Bullet weight is another obvious factor. A light bullet driven very fast produces higher RPM than a slower heavier projectile. Range (woods vs open country hunting) can also determine results. Provided the bullet does not lose integrity on its way to the target, it will gradually lose both heat and RPM downrange. Game size is a major consideration. As many of you know, the trends within the shooting sports are terribly woke now. The decline in shooting skills combined with ultrra short / light rifles was bad enough, but we are now seeing folk try to tackle very large animals with very small cartridges. This can be summarized as high expectations of equipment (and others for that matter) versus low self expectations. If I am not mistaken, this is indeed the definition of the word woke. Shot placement is a factor. In the past, men aimed to strike the shoulder to achieve fast killing (within a couple of paces). Nowadays, folk aim behind the shoulder and are quite used to seeing dead running game. They marvel at how small bullets driven fast create large wounds when used in this manner. Shoulder bones by contrast are very hard on thin jacket light weight bullets. I do advise against exceedingly high RPM numbers. It can make a rifle finicky and in some instances can cause mid air disintegration as was experienced in the past among 220 Swift users. Having said this, the newest sub caliber cartridges do require fast twist rates to optimize the BC's of the now very long projectiles. It simply is what it is. The new cartridges were initially designed for target shooting and then later marketed for hunting. These cartridges do have merit for hunting but one should maintain a level of common sense as to limitations. The current generation of lads at Hornady claim these cartridges are good for game up to the size of mature wild boar. Well yes and no. Neck shots with conventional -yes but not always. Frontal head shots - not always, some will deflect. Mono through the shield - yes at close ranges. Mono through the shield at distance - no it will run off and you won't find it. Conventual through the shield - no at all ranges and especially no if encrusted in wallow mud. Conventional behind the shield, yes but not if you aim too far back and no if wallow mud present. Pigs differ greatly in size and constitution depending on age and sex. And for some reason, nobody seems to discuss speed of killing these days. As a loose summary, game size is the number one factor. Most likely you will be fine using Hornady factory fodder, whether ELD-X or M so long as you are not tackling very large bodied deer, especially at woods ranges. Note that on light framed game, as velocity falls away, it is good to try to aim to break bone rather than deliberately aiming behind the shoulder. This helps to maximize the energy transfer at distance and minimizes dead runs. If an animal is so large that you have to aim behind the shoulder with a generic cup and core hunting projectile, the cartridge is likely too small. I do not wish to downplay the merit of premium bullets, but using a tough bullet does not always increase potential. To the contrary and with regards to the smallest bores, although penetration may be increased, a bullet which retains 95% of its weight reduces the transfer of kinetic energy from an already low energy yielding cartridge. Take it out hunting and see how you get on. Used within its limitations, you will likely thoroughly enjoy it. Hope that helps. All the best. |
@ 06:31 pm (GMT) |
Scott StruifRe: Twist rate and HUNTING bullet RPM speed limit?I read Howard’s question differently, more like, “Can a bullet go ‘splat’ when it hits game because it’s spinning too fast?” If you look at Hornady’s heaviest loaded ammo hunting offering for the 6mm CM, it’s the 103 grain ELD-X. They recommend, or at least tested it in, a 1 in 7 twist, 24 inch barrel. That barrel had a faster twist than Howard’s, and a muzzle velocity of 3050, presumably faster than Howard’s hunting rifle. I doubt Hornady would sell hunting ammunition they feared might go “splat.” That’s why they don’t recommend the ELD-M for hunting. They don’t want customers complaining that their bullets go “splat.” I’ve read that the 7mm RM was criticized, when it came out, as being so fast that bullets would go splat at close range. Seems like the critics missed the point. That’s what causes hydrostatic shock, as opposed to a hard bullet that penetrates without disrupting. |
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@ 05:39 am (GMT) |
Howard AudsleyRe: Twist rate and HUNTING bullet RPM speed limit?Yes.....a bit of both, but going "splat" was the major part of it. BTW, I also checked with Sierra and was told they see 280,000 rpm as about as fast as bullets ought to spin, but that was for flight integrity as much as anything. As far as I know, nobody has ever researched the "splat" factor........other than anecdotal evidence that some bullets explode on impact and do not penetrate into vitals. Getting to the bottom of what caused it almost impossible.As an observation, the current interest in long distance shooting.....while OK if limited to paper, gongs and such.......has NOT been good for hunting. Just because you can hit an animal at distance does not mean you can make a clean kill at distance unless you bring enough tool to do the job. The worst of the worst may be the guys who fell for the hype and are using the 6.5 Creedmoor on large game animals at over 500 yards. Even if they manage a decent shot that far, most likely result is a wounded animal. That is when the tracking starts. Most of the various 6mm calibers work well for us on our whitetail deer as most shots are no more than 100 yards. As one of the old guys in charge, I stress to the younger hunters if they want to stay in my good graces they need to leave the long distance stuff to paper and such. I want to see them field dressing deer, not tracking and losing wounded deer. But having said all that, I think that for our purposes no 6MM should have a twist faster than 8 and 8.5 to 9 would be better. A slow twist takes out the extreme top end of the target bullet line, but a too fast twist also takes out the bottom end......or at least may make them unusable for the intended purpose. |
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@ 12:16 pm (GMT) |
Howard AudsleyRe: Twist rate and HUNTING bullet RPM speed limit?BTW, for those wondering how spin rpm is calculated.......its......factor of 720 x velocity in fps then divided by twist rate So for a bullet with muzzle velocity of 3,200 fps down a barrel with 7.5 twist.... (720 x 3,200) then divided by 7.5 = 307,200 rpm Same velocity thru 8 twist barrel is 288,000 rpm The 6mm CM I have can get even a 103 ELD-X up to 3,200 FPS with room to spare......so to keep spin rate down to 290,000 rpm or so, I'd have to download, which seems silly to me. Again, I'm not sure if this is even an issue as far as integrity of an overspun bullet at impact is concerned. If anyone has a way to test it......I'd love to know. |
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@ 01:05 pm (GMT) |
Scott StruifRe: Twist rate and HUNTING bullet RPM speed limit?Hi Howard. The 6mmCM was designed for target competitors who wanted to shoot long boattail bullets with minimal recoil. Some of those competitors were using .243s with faster twist rates than the standard 1 in 10. The 6mmCM is an improvement over the .243, mostly due to its improved brass/chamber design. If you want to optimize it for deer hunting at 100 yards, don’t fret over twist rates. Choose bullets designed to shed weight but hold together enough to penetrate a shoulder. |
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