The Accurate Reloading Forums THE ACCURATE RELOADING.COM FORUMS Rifles Medium Bore Rifles Twist rate needed for deep penetrationGo | New | Find | Notify | Tools | Reply | |
| Twist rate needed for deep penetration | Login/Join |
| new member |
I have searched for info on this subject and can't find much of anything. I know that for certain length bullets in certain calibers, certain twist rates are needed to stabilize the bullet in flight. However, I have heard that for penetrating game instead of paper, more twist than what is required for stable flight is required. For instance, when the M-16 first came out, the .223 round it shot was said to be as deadly as the .308 because it tumbled on contact. When the twist rate was increased, this caused the bullet to not tumble anymore, correct? Does this principle hold for expanding bullets such as Core-loks and Partitions, or was it just valid for FMJ military type bullets? My 308 Win has a 1 in 11 twist, which according to Barnes is the minimum required for the 180 grain TTSX. Surely this is enough twist to allow for the world famous Barnes penetration. Anyone have any knowledge to share? | ||
|
| One of Us |
while twist rate relats to bullet stabilization, bullet construction, impact velocity and sectional density are factors to be considered in the realm of penetration. | |||
|
| one of us |
No. The bullet changes (weight & length) for M16 ammo were the reason for the twist rate change. The bullet change included a steel core in an attempt to afford body armour penetration. Cheers, Number 10 | |||
|
| one of us |
I think a sharp pointed FMJ .223 will still tumble in any twist. That said, you are at least partially correct in that if a bullet yaws any at all, it will have a negative effect on penetration as well as its actual BC. If it is slightly too slow a twist rate, a little yaw will have a dramatic effect on velocity loss over range and should limit penetration as well. I don't think the 1 in 11 twist will be any problem with a 180, even a TSX. I have seen a few .308's that were absolutely terrible in accuracy with 1 in 12's and 200 grain bullets however. A shot not taken is always a miss | |||
|
| One of Us |
I have never, ever heard that there was a coorolation between twist rate (spin) and penetration. How would that work? I certainly hope you're not using the fiasco called a M16 in .223 calibre as any sort of reference. Aim for the exit hole | |||
|
| one of us |
. | |||
|
One of Us |
Decent hunting bullets dont tumble when used like they where made to do. | |||
|
| One of Us |
Perhaps they should have done a little more studying BEFORE they put that POS in the hands of American fighting men. How many Americans died whilst the paper merchants were studying it and working out the bugs instead of saying in big letters: "We screwed up, let's start over and put a decent battle weapon and a decent, man killing cartridge in the hands of our troops." Aim for the exit hole | |||
|
One of Us |
IMO penetration is not related to twist rate except for the fact that the proper twist be achieved for stabilization of the bullet..... Focus first on stabilizing the bullet in flight and then on hitting the place you want to hit! /////////////////////////////////////////////////////////////////////////// "Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy, its inherent virtue is the equal sharing of misery." Winston Churchill | |||
|
One of Us |
I would suspect that twist rate is relevant to penetration only to the extent it affects bullet stability, as others have said above. I also suspect the more important factors include bullet shape and where the center of gravity is in the bullet, both when it strikes the surface of the animal and during penetration. Especially if it is an expanding bullet where the shape is changing during penetration. Lastly (for this post), I suspect that using higher twist rates can REDUCE penetration, after penetration has begun, though so little the efffect may be unmeasurable. Why? Because the more a bullet turns when in contact with a surrounding medium, the greater the potential friction on the skin of the bullet. More friction, theoretically at least, may decrease penetration. Of course, the exact opposite may occur too... a "buffer" flow zone may also occur, which could reduce friction. I've not heard of anyone even attempting to measure what pressures occur on the skin of the bullet during penetration of media such as flesh. Anyway, I don't even bother to try to worry about the twist rate affects on penetration. A bullet and twist proved in the field by general use is plenty good enough for me. My country gal's just a moonshiner's daughter, but I love her still. | |||
|
| One of Us |
Yes, spin rate does effect penetration. When 15 years old I bought a 7MM Weatherby Magnum. It had a 26-inch barrel with a 1-in-12 twist. I shot coyotes with it using 120-gr and 139-gr Hornaday bullets. Exit holes were 1-1/2 inch in diameter or so. Kids told me how their fathers shot coyotes with .270s and .30-06 and the exit holes were 3 or 4 inches in diameter. I thought they were exagerating. Eventualy I burnt the barrel out and had the gun rebarreled with a barrel with a 1-in-9 twist. The coyots shot with it generally had 4 inch exit holes. Then next time it was rebarreled it was with another fast twist barrel and finally the last time was again with a 1-in-12 twist barrel in 7MM STW. The final barrel would shoot 120-gr Hornaday hollow point completley through antelope and produce exit holes of less than 2 inches. Anyway, over the years there were a lot of coyotes and maybe 25 antelope shot with that rifle and a slow twist barrel does produce smaller wound channels and deeper penetration. | |||
|
| one of us |
I. | |||
|
| One of Us |
From my understanding of FMJ spitzer style bullets used in military applications they all yaw after impact. However bigger heavier ones penetrate deeper before they do it. The early M16's used a slow twist and light bullet. This combination let the bullet yaw sooner on impact and they were also faster due to being lighter than many of the bullets used now so they also fragmented at closer ranges. Then the powers to be felt we should make the bullet heavier, barrels shorter and make it penetrate Kevlar helmets at 600 yards. Well now you get more poke thru's on un armored skinny enemys like we face in the sand box. Just do a search and study the wound channels the 5.56 can inflict when loaded right. It don't make up for a hard hitting .308 but it is impressive. But the 5.56 can't be loaded the same for the M4, M16 and Saw and be expected to work well. Molon Labe New account for Jacobite | |||
|
One of Us |
Twist rate??? Penetration??? I'll let you know later...I'm looking for my wife to try out something new. ________________________________________________ Maker of The Frankenstud Sling Keeper Proudly made in the USA Acepting all forms of payment | |||
|
One of Us |
I'm with IOWADON on this. Compare drilling a smaller sized hole though steel or wood. For the amount of force behind the drill, a fast spinning drill bit will penetrate faster and easier than a slow turner. Same would apply in flesh and bone, as the bullet mushrooms and develops cutting petals, the bullet spinning faster will likewise penetrate easier and quicker and therefore deeper for the given momentum of the bullet than a slower spun bullet as IOWADON has found in practice. | |||
|
| One of Us |
Not if you are drilling a foot deep every revolution of the drill! /////////////////////////////////////////////////////////////////////////// "Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy, its inherent virtue is the equal sharing of misery." Winston Churchill | |||
|
| One of Us |
I don't know that faster twist rates specifically aid penetration. But.... It is true that when the M16 in 223 with 55 gn bullets was first adopted that the 1:14 was marignally stable which caused the bigger wounds from the tumble on impact. They switched to 1:12 to increase stability in colder climates, which negated that devastating wound capability. Then they upped the velocity to try and make up for it; one of the many band aid repairs from deviating from the original design through the years. I won't beat that horse to death here. PA Bear Hunter, NRA Benefactor | |||
|
| One of Us |
I think this is the main reason for switching away from the 14" twist. Frangible bullets break up at the high velocity of the 5.56 mm or .223 Rem. I would imagine they shatter before they can tumble. Both would cause ugly wounds to human beings, but then again I am not a doctor. I am sure Alf can educate us here, it is his speciality. Warrior | |||
|
| One of Us |
This is especially true if you load your ammunition with drill bits rather than bullets. I think I gotta call BS! Don't ask me what happened, when I left Viet Nam, we were winning. | |||
|
| One of Us |
In terms of in-target stability of a bullet it comes down to the bullet's geometry and construction, and not on a higher SF values taken at the muzzle (Sg increases over distance in any event) Warrior | |||
|
| One of Us |
 HUHThats a wild one-id say nuts and BS but its so far out I want to see references. SSR | |||
|
| One of Us |
Very graphic photos plus some discussion from a few who understand how FMJ bullets yaw. http://www.timawa.net/forum/index.php?topic=17111.0 Molon Labe New account for Jacobite | |||
|
| One of Us |
This I am coping from somewhere but the guy explains this topic very well and it is in line with my research on the matter. This post is based on fact, not more Vietnam-era myth that, to many people's annoyance (including mine), persists to this day. As a general rule, any negative comment about the M16 or 5.56 NATO that comes from the Vietnam-era is almost always bullshit and easily proven wrong or explained somehow. The military personnel are the greatest source for these unsubstantiated claims, and unfortunately, their stories often need to be taken with a grain of salt as they were often passed from one soldier to another, perpetuating the myths. Fact: there is no such thing as a one-shot stop anywhere other than to the T-Zone of the head. Men have been shot repeatedly with .30 caliber bullets, .50 caliber bullets, and in some extreme cases, 20mm bullets and STILL SURVIVED. I knew one man who was shot repeatedly on 3 seperate occasions in Vietnam by AK-47s and SKSs. Yet, he lived to show the numerous scars. THIS DOES NOT MEAN .30 CALIBER BULLETS ARE NOT EFFECTIVE! It ONLY means that there are MANY factors which determine whether a bullet kills someone! Why the 5.56 NATO continues to be villanized, I have no clue. All I know is, it gets fucking old. I suspect if the government had done more testing with the AR-15/M16 before they sent it sight unseen, after Colt's mods, to Vietnam, we wouldn't be debating this. Now, I'm NOT bashing military personnel. All I'm saying is, a soldier's understanding of ballistcs and even gun function can usually be measured on the head of a pin. They are making unqualified claims whey they repeat most of which they say about these subjects. The perfect example is the "M16 jams up all the time" myth which started in Vietnam. As I hope you all know by now, this was because of three things: Colt tightened the original specs of the rifle, the ammunition fouling turned to laquer in moisture and our soldiers weren't cleaning the guns properly. The cleaning problem persists even today, as many of the soldiers in our military are lazy, don't pay attention, are stupid, etc. Take your pick. Anyone who has been in the military knows it's chock full of dum-dums. And while I wont argue that a good combat weapon should require a minimum of cleaning (this is a different topic altogether), even today many problems with the AR/M16 platform can be explained by very poor maintenance or other operator error. The truth is, the AR-15/M16 has never escaped it's early, fatefull beginnings, and it probably never will as long as uneducated myths continue to propagate and as long as those who don't do their own research continue to take these myths hook, line and sinker. In other words, don't fault the gun for your or anyone else's fuck up. But I digress... Here is the real story about slow twist rates, yawing bullets and nasty wound channels: When the M16 was first used in Vietnam, it was assumed that the smaller 5.56mm round would make much smaller wounds than the 7.62mm M80 round fired from the M14. Everyone was surprised to learn that M16 wounds were often much more severe. In order to explain this discrepancy, it was theorized that the slow 1:14 barrel twist made the bullet less stable in flesh and caused it to tumble, resulting in the large wounds. In fact, the slow twist only made the bullet less stable in air. Any pointed, lead core bullet has the center of gravity aft of the center of the projectile and will, after a certain distance of penetration, rotate (yaw) 180° and continue base-first. This is where the appearance of "tumbling" came from. Fact: Flesh is as much as 1000 times denser than air and will cause a bullet to lose stability almost instantly. For M193 and M855 ammo, this typically occurs after 3-5 inches of flesh penetration, though this can vary. In order to spin the bullet fast enough to be stable in flesh, the barrel twist would have to be on the order of 1 twist every 0.012 inches, which would look like the barrel had been threaded instead of rifled. The actual cause of the larger-than-expected wounds was not a result of this yawing of the bullet, but of the velocity of the bullet coupled with the bullet's construction. M193 bullets, the ones used in Vietnam, have a groove or knurl around the middle, called a cannelure. This allows the mouth of the case to be crimped on to the bullet, preventing the bullet from being pushed back into the case during handling and feeding. The cannelure also weakens the integrity of the bullet jacket. When the bullet struck flesh at a high-enough velocity, the bullet's thin jacket, weakened by the cannelure, could not survive the pressure of moving sideways through the dense flesh. Instead, the bullet would only rotate about 90°, at which point the stresses were too much for the bullet jacket and the bullet would fragment. The results were a wound that was far out of proportion to the size of the bullet. Yet, the twist rate of the barrel and therefore the rotation speed of the bullet, is not a factor in the fragmenting equation. M855 ammo works exactly the same way, though due to its heavier bullet, it has less muzzle velocity. Less muzzle velocity translates to a shorter range in which the bullet retains enough velocity to fragment, compared to M193. The importance of rate of twist in wounding is a frequent subject of what we politely call "ballistic myth." Any projectile that has a "center of pressure" forward of the center of gravity will tend to tumble. You can illustrate this to yourself by trying to balance a pencil on your fingertip. Spin, given to the projectile by barrel twist, puts a projectile into a state described as "gyroscopically stable." The projectile might be momentarily disturbed but will return to nose-forward flight quickly. To describe how stable a given projectile is we use the gyroscopic stability factor (Sg). Generally you want a factor of 1.3 or greater for rifle rounds. 1.5-2.0 is a generally accepted value for 5.56 rounds. For M193 the following variables apply: axial moment of inertia (A) = 11.82 gm/mm2 transverse moment of inertia (B) = 77.45 gm/mm2 mass (m) = 3.53 grams reference diameter (d) = 5.69 mm Using the gyroscopic stability formula: Sg = A2 p2 / (4 B Ma) and assuming sea level we use an air density of 1.2250 kg/m^3 and discover that this this projectile will need on the order of 236,000 rpm for good stability (Sg > 1.3). At 3200 fps M193 is typically spun up to more like 256,000 (1:9" twist) to 330,000 rpm (1:7") so that Sg approaches 1.9 or 2.0. 1:12" rifles will spin rounds at around 192,000 rpm and 1:14" rifles around 165,000 rpm. You can see why 1:14" rifles might have had trouble stabilizing M193 rounds. Clever math types will see that density of the medium traversed (air in this case) has a dramatic effect on the spin required to maintain the Sg (density being in the first term's divisor). This is why cold conditions tend to dip "barely stable" rounds below the stability threshold. Without doing too much calculus it will be seen that an increase of three orders of magnitude (1000) in this variable will be a dramatic one for spin requirements. To balance things spin must be increased to compensate. Through human flesh (which varies from 980 - 1100 kg/m^3 or about 1000 times the density of air) something on the order of 95,000,000 - 100,000,000 rpm is required to stabilize a projectile at speed. Given these differences it will be seen that the difference between a 1:12 or 1:14" twist when it hits flesh and a projectile launched from a 1:9 or 1:7" weapon is so small as to be beyond measuring. But the game isn't over yet. Gyroscopic stability of 2.0 or so is sufficient for a M193 projectile to recover from an upset quickly, return to nose-forward flight and not be over stabilized. To prevent the upset in the first place, particularly when a sudden and very extreme change in density (and therefore drag and pressure applied to the center of pressure) requires FAR more stability. To grant enough stability force to prevent the upset of a M193 projectile encountering a sudden 1000 fold increase in density a factor of as much as 10 to 50 times (speaking VERY conservatively) the required gyroscopic stability for a steady state flight through a medium of that density would be required. In other words, unless the projectile is spinning at nearly a BILLION rpm it is going to be upset by such a transition. Even at this rpm it is like to be upset somewhat. In summary, and to take the most extreme case, a M193 projectile spinning at 350,000 rpm (from a 1:7" rifle) is going to upset in flesh (yaw) exactly as fast as one spinning at 150,000 rpm (from a 1:14" rifle). Claiming that twist rate has any impact on the speed of yaw and therefore terminal performance is just not in line with the laws of physics. Molon Labe New account for Jacobite | |||
|
| One of Us |
The bullets used where I noticed small exit holes for a slower twist rate were Hornaday 139-gr SP, Hornaday 120-gr SP, and 120-gr HP (all 7MM). These were all at a very high muzzle velocity and I think all these bullets were not just mushrooming, but fragmenting. Picture two bullets moving at equal velocities with either a slow spin rate or a fast spin rate. Now picture the slower spining bullet turning from a solid into a dozens of pieces of copper and lead. The result would be a cone shaped path of the pieces. Now picture the faster spining bulet doing the same. The faster pieces of the faster spining bullet would also turn into a cone shaped path, but the cone would get wider faster. I think that is what happens on impact with game when a very fragmentable bullet is used. I've read about thin jacketed bullets blowing up after exiting the barrel if pushed too fast and I think centriphical force is large factor in bullet performance. For example, look at two identical bullets shot at the same vlocity but with different twist rates. One shot from a 1-in-9 barrel would be spinning 33 percent faster than one shot fron a 1-in-12 barrel. Since centriphical force is proportional to the square of the spin rate, the centriphical force on the bullet with the 1-in-9 twist would be 77 percent more than the one with the 1-in-12 twist. I think that is in cases of fragile bullets an important factor on their performance. | |||
|
| One of Us |
As I stated earlier I am not going to expand on the band aid repairs done after the Army deviated from the original specs of the AR-15. My reference is The American Rifleman January 1968, when I first be came a member, incidentally. I don't knock the M16; I am retired Army; but whgat is true is true. PA Bear Hunter, NRA Benefactor | |||
|
One of Us |
My thoughts EXACTLY!!!    I suppose AA or AAA makes a difference? | |||
|
| One of Us |
You have some things wrong. Let's start with the problems of early fielding the M 16 in Nam before the bug were thoroughly wrung of the rifle. First off McNamara would release the money to have the bores and chambers chromed. Kind of stupid considering the climate they were fighting in. This resulted in deeply pitted and corroded chambers which isn't exactly good for case extraction. The 5.56 was developed to run with stick powder, namely DuPont IMR series. Next the government switched to Winchester ball powder. That's not so so bad, but was was that Winchester put way too much calcium in the powder to create a long shelf storage life. This resulted in plugging gas tubes and along with the pitted chamber (remember they weren't chromed) and the dirty burning characteristics of that ball powder made the M16's life miserable. Next it's been told that Frankfort Arsenal annealed the cases wrong resulting in too soft of a case. That was great for the extractor to pull through the rim because the chambers were pitted along with the dirty carbon from the over calciumed Winchester powder which wasn't the powder that was supposed to have been use. Throw in that in the first days the soldiers were told it was a wonder rifle that never needed to be cleaned and they even weren't issued cleaning equipment for it. Second it was proven out that the 14 twist was too slow to stabilize the 55 grain FMJ bullet and I believe this came before any rifles even went to Vietnam. I believe it was the Air Force that stumbled upon this first. It was reported to Stoner and the twist was changed to 12 right away. So if you can actually find an early M 16 with a 14 twist you can become a very rich man as they are as rare as hens teeth. This also perpetuated the notion that some M16's went to Nam that were smooth bored to deliberately tumble the bullet. That's not true either. Okay, I have a Naval doctor friend who is a ballistics forensic expert who mainly studies battle wounds. He and I fight constantly over him saying a 55 grain bullet fired from a 7 twist does no more damage then the same fired from a 12 twist, in fact stating less. I've pointed out to him shooting animals ranging from ground squirrels on up through large dog size and the 7 twist showed much more damage then the 12 twist with the same bullets. He merely stated that human bodies cause to bullet to react differently. I do agree with that the 55 FMJ doesn't destroy itself in flesh and it does tumble and the boat tail cases that mainly. A flat base won't tumble as much. | |||
|
| One of Us |
Yeah, that's why it's been our country's rifle for over 45 years and still going on yet. Also it's one of most sought after military rifles in the world. Too bad you had a bad experience with it or believe all the BS on the internet or heard about it. | |||
|
| One of Us |
Smokinj, I only posted what someone else had wrote some place else so I actually had nothing wrong. Plus I saw no reason to add about the powder or chrome lining. It has nothing to do with the twist of the rifle. Molon Labe New account for Jacobite | |||
|
| One of Us |
 | |||
|
| One of Us |
No, but it had a lot to do with all the bullshit stories about the M16's. | |||
|
| One of Us |
DING, DING, DING, DING, DING. We have a winner. And shooting watermelons at a picnic for some generals does not constitue "testing" regardless of what any of the Kennedy's Harvard boy might say. | |||
|
| One of Us |
Ding Ding Ding...you're wrong too! Research it, show me where they used a 14 twist in the M16 in Nam? Guess what too, all bullets yaw to some degree. | |||
|
| One of Us |
Forget the 14 twist. If you look at my original statement I say all fmj spitzer style bullets will yaw. Heavier ones just take longer. The guy I quoted said the 14 twist made it to Nam. Given a large enough medium the .308 fmj will yaw but in the human body it is usually out the other side before it has a chance. Arguing about what twist made it to Nam really has nothing to do with the topic. My answer to the penetration question is the bullet has more to do with penetration vrs tumble(Yaw) and expansion than rate of twist. Molon Labe New account for Jacobite | |||
|
| one of us |
. | |||
|
| One of Us |
Defense Secretary Robert McNamara ordered the change in the twist rate. The damage caused by the .223 (5.56mm) "varmint" bullet was observed and originally believed to be caused by "tumbling" due to the slow 1 in 14 inch rifling twist rate. However, this twist rate only made the bullet less stable in air. The large wounds observed by soldiers in Vietnam were actually caused by projectile fragmentation, which was created by a combination of the projectile's velocity and its construction. Warrior | |||
|
| one of us |
Yup - +1!  Cheers, Number 10 | |||
|
| One of Us |
Well, someone is on track somewhat. The 14 twist was chosen, so as to impart to the carefully designed bullet, exactly the correct degree gyroscopic stability to maximize it's lethality. On the other hand, it was hardly without precedent. Rifles being built by Remington for the .222 Remington commercial cartridge had a 14 twist of rifling, as indeed did nearly all other "varmint" rifles firing .22 caliber bullets of about 50 to 55 grains in weight. The gyroscopic stability of 55 grain "varmint" bullets was quite satisfactorily in the 14 inch twist, and at first thought it might be supposed that the 14 twist would impart the same degree of stability to a 55 grain military bullet as well. In any event , the 14 twist failed to provide sufficient spin for adequate gyroscopic stability of either Stoner's original bullet, or the M193 bullet designed by Remington. It was the Air Force that first discovered this in cold weather testing of the M 16. It was brought to everyone's attention, but at first the Army was totally against changing the twist, then later going along with it. Yes McNamara approved the change, after all he held the money. He knew shit about firearms. | |||
|
| One of Us |
Uhhh, Commandant Marine Corp issued a letter to all Marine officers that ANY negative reports on the M16 would bring discipline. Not exactly what you'd call a heartfelt endorsment. You're right, whole lot of bullshit on the internet and I'm glad to see you adding your fair share. Aim for the exit hole | |||
|
| One of Us |
The first 1000 rifles ordered by DARPA and issued to the ARVN had a 1-14 twist. I believe the second contract of 19k to the Airforce and 85K to the Special Forces also had the 1-14 twist. For following contract of 850K+ rifles for non SF troops had both the forward assist, and the 1/12 twist barrels. But prior to this I never referenced the twist rate either, just that all FMJ bullets will yaw. But I think this only makes me the 6th or 7th person to point this out.... | |||
|
| Powered by Social Strata | Page 1 2 |
 | Please Wait. Your request is being processed... |
|
The Accurate Reloading Forums THE ACCURATE RELOADING.COM FORUMS Rifles Medium Bore Rifles Twist rate needed for deep penetration
Visit our on-line store for AR Memorabilia