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Old June 24, 2010, 08:23 AM   #1
Magnum Wheel Man
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Has there ever been studys done on twist rate vrs bullet terminal damage ???

my local shooting mentor / buddy & gunsmith & I have engaged in this conversation several times...

terminal "damage" seems to have so many factors...

bullet construction
bullet diameter
bullet velocity

but both of us have seemed to witness a higher "destructive" power with faster twist barrels, with as many of the other factors as equal as possible... curious as to you're thoughts...
& if anyone has any references as to bullet rotation vrs damage???
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Old June 24, 2010, 08:35 AM   #2
jman841
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I have actually heard the opposite, however, dont have any evidence to back it up, but a bullet that is just barely stabilized will destabilize rapidly when hitting a target as opposed to a heavily or over stabilized bullet. Supposendly one of the reasons the original M16 was supposed to be effective. 55 grain round at 32-3300 FPS with a 1/14 twist. not very stable.
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Old June 24, 2010, 08:52 AM   #3
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It seems like the more stabilized the bullet the better the penetration.

Which may or may not be true. But if it is, then it would depend on the target in question and how much penetration you want.
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Old June 24, 2010, 08:58 AM   #4
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It all depends on what type of bullet is being used.

Yes, if a bullet tumbles on impact, more damage will be the result-while sacrificing accuracy at longer ranges.

Now, let's say you have a 77gr HP coming out of a 1:7 twist barrel. You would assume that once the bullet impacts soft tissue, it would have a "drill" affect, and as the bullet fragmented, you would have razor sharp shrapnel still spinning until all momentum is lost.

I think, if you're pushing high velocities, you're going to see a bullet tumble in soft tissue. Whether the twist rate is fast or not. A bullet traveling 2400 to 3200 FPS and hitting bone, or some tough soft tissue, should deflect some, and casue a bigger wound channel-that is if you're using lighter weight bullets.
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Old June 24, 2010, 09:14 AM   #5
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I would disagree with a slow twist rate degrading accuracy if the bullet weight is balanced to the barrel. A 223 firing a 55 grain will easily stabilize in a 1 in 12 and be very accurate. A 220 swift will stablize its bullets in a 1 in 14 twist rate on up to 63 grain projectiles and maintain excellent accuracy.

In contrast I have seen high twist rates like 1 in 7, deform and even tear apart bullets as they left the barrel if they were to thinly constructed. i.e. something like a 52 grain match bullet.
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Old June 24, 2010, 09:38 AM   #6
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Quote:
bullet rotation vrs damage
I did the calculations once and found that rotational energy is only a tiny, essentially insignificant percent of the ME - certainly not enough to damage tissue on its own: If the bullet could spin without traveling forward, you'd be able to stop the bullet from spinning with your fingers without damage to them. I doubt there's much spinning of the bullet or shrapnel going on after the initial impact on soft tissue.

My guess is that, to a first approximation, it's generally just a matter of heavier bullets being used with higher twist.
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Old June 24, 2010, 10:09 AM   #7
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If you look at photos of hunting bullet impacts inside ballistic gellatin, you can see the effects of the bullets spin causing distructive damage. With the bullets expansion on impact thowing energy out in a twist
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Old June 24, 2010, 10:28 AM   #8
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1+ baltz526, I seen a high speed video of balistic gel and the block almost twisted over.

But at the same time I would think that some counter torque would be experienced in the rifle
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Old June 24, 2010, 10:38 AM   #9
Bartholomew Roberts
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Quote:
a bullet that is just barely stabilized will destabilize rapidly when hitting a target as opposed to a heavily or over stabilized bullet.
A mostly liquid medium like flesh is much, much denser than air. There is no modern rifle made that induces the kind of twist rate necessary to spin-stabilize a spitzer bullet in flesh. This link goes into it in more detail.

Now where the nose of the bullet is pointing at impact has been shown to have an effect on how rapidly it destabilizes (speaking purely of yawing and fragmentation of 5.56 FMJ ammo in ballistics gel), so to that degree I suppose you could make an argument that twist rate can effect how soon a bullet yaws; but short of having the round overstabilized to the point the nose points up, I don't see how you could control where the nose was at the point of impact given all the variables.
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Old June 24, 2010, 10:40 AM   #10
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the twist rate of the barrel/ spin of projectile has very little to do with damage. there might be a very loose corrilation at best, but nothing of signifigant note.

the spin imparted by the barrel's twist is almost entirely about stabilization of flight. too low a spin and wobble is introduced witch will affect accuarcy. too much spin and the projectile will litterally tear it self appart, never reaching the target.

bullet spin helps the target reach the target, but much like the spoiler on a nascar/indycar there are many more factors in winning the race, or in this case killing the target.
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Old June 24, 2010, 12:09 PM   #11
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Quote:
If you look at photos of hunting bullet impacts inside ballistic gellatin, you can see the effects of the bullets spin causing distructive damage. With the bullets expansion on impact thowing energy out in a twist
Quote:
1+ baltz526, I seen a high speed video of balistic gel and the block almost twisted over.
Not to quibble, but I'm thinking that's not the effect of twist energy, but possibly the forward energy of the bullet as it veers from a straight line once it hits the gel.

If the bullet were shedding enough rotational energy to cause damage to gel or soft tissue, the bullet itself would be under considerable rotational stress, but check out this hollow point .50S&W bullet recovered from gel:

http://www.jturkoski.com/Home%20Page.../50-magnum.htm

A beautiful rosette, with no rotational deformation of the leaves.

Just to be sure, I re-did my calculations for a 77 grain .223 bullet shot from a gun with a 1:9 twist (general details below). Maybe someone can check these calcs? At any rate, this round is predicted to have a MV of 2750 fps and ME of 1293 ft-lbs, yet the rotational energy comes out to a mere 0.1 ft-lbs. This is equivalent to a 10 grain air gun pellet traveling at only 67 fps.



General calculations (all units converted to SI):

Erotational = (I w^2)/2, where:

I = moment of inertia = (m R^2)/2 for a solid cylinder of radius R.
w = rotational speed in revolutions per second

For a 77 grain bullet with diameter 5.69mm:

I = 20.3 x 10^-9 Kg M^2

w = 3667 revs per second

Therefore, Erotational = 0.136 N M^2/S^2 = 0.136 Joules = 0.1 ft-lb.


As a double check, I also did the same calcs for translational (i.e. muzzle) energy, and got precisely 1757 Joules or 1293 ft-lbs.

http://en.wikipedia.org/wiki/Rotational_energy
http://en.wikipedia.org/wiki/List_of_moments_of_inertia
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Old June 24, 2010, 12:44 PM   #12
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On the contrary, slower twist barrels produce more destructive terminal results. I'll explain:

A rifle round destabilizes when it enters human flesh because it is far more dense than air (the medium which is accounted for when calculating barrel twist). The bullet's rotation speed is greatly reduced very fast and once it isn't rotating fast enough it tumbles because it's center of gravity is toward the butt of the bullet. For 5.56 rounds and within about 100 yards, the round will fragment once it tumbles 90 degrees.

So with a slower twist rifle, the round destabilizes faster in flesh. I'll use 5.56 as an example: lets say 1:7 twist requires about 6 inches of flesh to slow down before it tumbles. A 1:9 would then be roughly 4 inches. 1:12 would be even shorter.
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Old June 24, 2010, 01:29 PM   #13
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One perspective to look at this:
If we assume a 10 inch thick target,and a 1 in 10 twist,and ignore decelleration,the bullet would make one revolution going through the target.
Of course,both forward speed and rotation speed decrease,and decrease more rapidly with expansion.
IMO,I do not see a buzz saw going through the target,even a tight twist might only get two revolutions.
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Old June 24, 2010, 01:41 PM   #14
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Ther are an awful lot of people's opinions, "thinking", and "reasoning" here, and very little actual data. Very little scientific research has been done on the subject, as far as I know.

As far as non-scientific research, I actually knew and spoke at great length with a man who did quite a bit of testing on this subject back in the 1950s. He was a wildcatter and gunsmith who worked very closely with the Juenke brothers of Saturn Guns fame, as well as PO Ackley, Frank Barnes (who wrote COTW), and other well-known gun gurus, and who mentored Wes Ugalde (of TCU fame). This data was gathered by shooting animals, not paper, and was based on shooting hundreds of animals, not just one or two. IIRC, he experimented with 30-06, 270, 308, 7mm-06, 243 Win, 230 Ackley (80 gr .228" bullet on an -06 case), 22-caliber wildcats, and a few 17 caliber wildcats, using Nosler Partitions and hand-swaged bullets. His experience indicated that a higher rate of twist gave deeper penetration and considerably higher damage than slower rates of twist, but required better built and heavier bullets than factory bullets to really shine. Higher rates of twist gave deeper penetration (which always seems better than less penetration), longer wound channels (which seems better than short wound channels), and a very high percentage of 1-shot kills.

This is consistent with a lot of folk knowledge, including Roy Weatherby's claims of outlandish performance for his rifles (257 Weatherby and 300 Weatherby in particular), PO Ackley's claims of terminal ballistics, Mashburn's cartridge development, and Warren Page's work. It also is supported by the killing abilities of seemingly underpowered rounds that killed out of proportion to their paper credentials, like 22 Savage High Power, 6.5X54mm MS, and 7X57, which had higher rates of twist than needed launching heavier bullets than we would normally use here in the USA.
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Old June 24, 2010, 02:52 PM   #15
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to be fair... as unscientific as it was...

his exprience was with crows... mine with p-doggies... & 223's specifically with my expirience... & for evidence, it was the amount of spectacular "spray" of parts so we are not talking human or even game sized animals... but it would make sence, that as the bores went up, the results would follow

talking 20" barrels, Contender 1 in 12" ( or 14"... I forget which ) twist, vrs AR with a 1 in 9 twist, & the same handloads with 50 grain bullets...
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Old June 24, 2010, 04:05 PM   #16
Bartholomew Roberts
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Quote:
On the contrary, slower twist barrels produce more destructive terminal results. I'll explain:
On the contrary, you're wrong. If you'll read the link provided earlier, it will show you the math describing this.

Quote:
A rifle round destabilizes when it enters human flesh because it is far more dense than air (the medium which is accounted for when calculating barrel twist).
Yes, so much more dense than air that the difference between a 1:14 twist and a 1:7 twist is like the difference between an elephant and a elephant with a grain of sand on its back. You would need something on the order of a billion rotations per minute to keep a bullet stabilized as it transits from air to flesh.

Quote:
So with a slower twist rifle, the round destabilizes faster in flesh. I'll use 5.56 as an example: lets say 1:7 twist requires about 6 inches of flesh to slow down before it tumbles. A 1:9 would then be roughly 4 inches. 1:12 would be even shorter.
This is also provably wrong from an observational point of view as well. You can look at gelatin tests of the Hornady 75gr TAP round out of both 1:7 and 1:9 barrels and the rounds upset at pretty much the same distance regardless of twist.
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Old June 24, 2010, 04:08 PM   #17
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I think there is a article in the recent edition of Barnes Reloading Manual.
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Old June 24, 2010, 08:11 PM   #18
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I don't think twist rate has a whole lot to do with terminal ballistics. If it did, a wide receiver would have trouble catching a football and would have a very hard time adjusting between left and right-handed quarterbacks. IMHO, of course.
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Old June 24, 2010, 10:15 PM   #19
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If I may add a little more confusion;

A spitzer fmj with a higher twist is likely to have more yaw and this will cause it to tumble quicker in flesh. On the other hand a decent soft point bullet will mushroom on impact or soon thereafter and the mushroom creates a temporary vapour shield around the remaining bullet shank thus preventing it from tumbling - providing it does not strike bone or something.

As for bullet spin - well, I have fired bullets into rags and wet rags and they often captured the fibres and teisted them up around itself and often stopped pointing nose forward. BUT, these were soft points that had mushroomed.

It is true that the rotational energy is relatively small but it also concentrated. Grab a spinning bullet and it's going to go from a few hundred thousand RPM to zero in a short space of time - it's gonna take some flesh with it. The energy may be low but the power could be very high. Power is RATE OF ENERGY TRANSFER. Spread that 'power' over the length of the wound channel it becomes insignifcant.

There was no noticable difference between my 1-in-16 twist hornet firing a 60gr spire point soft nose and a 1-in-12 223 firing a 55gr bullet of stronger contruction when shooting hares. Both bullets produced large exit wounds with lots of blood and flesh being sprayed out.
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Old June 24, 2010, 11:05 PM   #20
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Quote:
Originally Posted by MrBorland
Erotational = (I w^2)/2, where:

I = moment of inertia = (m R^2)/2 for a solid cylinder of radius R.
w = rotational speed in revolutions per second

For a 77 grain bullet with diameter 5.69mm:

I = 20.3 x 10^-9 Kg M^2

w = 3667 revs per second

Therefore, Erotational = 0.136 N M^2/S^2 = 0.136 Joules = 0.1 ft-lb.
One small thing: angular speed w must have units of rad/s instead of rev/s for the subsequent equations to be true.

1rev = 2π rad which, dividing by rev, leads to 1 = 2πrad/rev

w = 3667rev/s * 2πrad/rev = 23040rad/s

Then repeating KEr=(Iw^2)/2, we get 5.39J, or 3.97ft-lbe, which is still over two orders of magnitude less than the bullet's muzzle energy and is insignificant.

That number is higher than actual, because an ogive-shaped bullet will have a moment of inertia 2I<mR^2.

Between 0 and 100yd, the bullet loses an average of about 2.2ft-lbe every yard it travels (using data for 55gr FMJBT from this table). So if the target takes two large steps backward, the kinetic energy advantage of a spinning bullet relative to a non-rotating one is totally negated.

Last edited by LukeA; June 24, 2010 at 11:10 PM.
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Old June 25, 2010, 09:51 AM   #21
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Thanks LukeA for the clarification.

Quote:
Grab a spinning bullet and it's going to go from a few hundred thousand RPM to zero in a short space of time - it's gonna take some flesh with it. The energy may be low but the power could be very high. Power is RATE OF ENERGY TRANSFER. Spread that 'power' over the length of the wound channel it becomes insignifcant.
Good point. You might (or might not) would lose some skin if you stopped it rotating with your fingers. Subject to LukeA's review, I calculate that spinning 77 grain .223 bullet would release about 20,000 watts of power to your fingers (assuming it only takes 2 rotations to stop). OTOH, that'd be released in only 0.5 milliseconds, and I don't know how much heat skin can absorb in that short period. Copper transfers heat very well, so my guess is that the jacket itself would absorb most the the heat.

As far as relative damage to soft tissue, while the rotation of the bullet may release 20,000 watts, the forward motion releases about 4 million watts (assuming 12" penetration), so even WRT power, as 303Guy suggests, the rotational component is negligible.
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Old June 25, 2010, 10:00 AM   #22
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so... could / does this additional rotational force, cause a more dramatic "expansion" of the bullet on impact, if the bullet is of the design ( like a varmint bullet ) to rapidly come apart ??? I can understand that a good soft point hunting bullet isn't going to "explode" on impact, but perhaps the additional rotation of a faster twist barrel helps those easily frangable bullets come apart more dramitically on impact ???
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Old June 25, 2010, 12:52 PM   #23
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Quote:
so... could / does this additional rotational force, cause a more dramatic "expansion" of the bullet on impact, if the bullet is of the design ( like a varmint bullet ) to rapidly come apart ??? I can understand that a good soft point hunting bullet isn't going to "explode" on impact, but perhaps the additional rotation of a faster twist barrel helps those easily frangable bullets come apart more dramitically on impact ???
Let's say that we have a stationary, non-rotating bullet. Let's also say that filling this stationary bullet full of smokeless powder with total chemical potential energy equal to the bullet's rotational KE in flight is a decent simulation of the bullet stopping rotating under extremely high negative angular acceleration.

By my calculation (using 4.62MJ/kg as the energy density for smokeless powder), the explosive effect of the spinning bullet coming to a stop very rapidly are equivalent to filling a small cavity in the bullet with 0.018gr (grains) of powder. That's about 1/10 as much powder as is in one of those party poppers where you pull the string and little streamers shoot out. I don't think the effect will be significant.
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Old June 25, 2010, 01:04 PM   #24
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Seems like there should be some way to test this with a smoot vs rifled bore gun. Of course I guess the bullet shape itself would have to be different, so it would probably not work.
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Old November 19, 2013, 12:37 AM   #25
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The relationships between twist rate and bullet weight

This is not by any means a comprehensive look at the problem but it does examine the effect of twist rate on certain military ammunition. Essentially, the .223 round does more damage when it yaws upon impact and tumbles inside the body. It's more likely to do this when the bullet is just barely stabilized by the barrel's twist rate. You can read the details at http://www.ballisticstudies.com/Know...Remington.html. Hope you find it as interesting as I did.
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