Twist rate and penetration: also factors effecting penetration and wound channels in general

[Marbles]

So, obviously too slow of a twist will decrease penetration and increase deflection. But, will over stabilizing a quality, heavy for caliber, expanding bullet decrease deflection on bone?

That is the question, and feel free to answer without reading more. I'm interested in data/science more than conventional wisdom, but conventional wisdom is welcome too.

I have heard increased twist rate results in larger wound channels for expanding projectiles given increased energy pushing the projectiles mass outward from center. The increased rotational energy also means more energy stored in the projectile that can be transferred to the target.

Will a marginally stabilized 180 gr 308 be more or less likely to deflect with a low angle strike against bone than say a well stabilized 75 gr 223?

Obviously mass plays a roll due to momentum being what resists change in an objects movement.

Bullet construction also obviously plays a roll, and extrapolating from Hornady TAP data soft tipped bullets will deflect less

I suspect sectional density plays a roll as it concentrates that momentum and decreases the lever arm of the mass located on the projectiles outer circumference.

 

[Marbles]

I'm guessing a 178 gr ELD-X is a bad choice from my Tikka 308 with 1:11 twist given a sea level, 60 F stability factor of 1.35. The same bullet from my 1:10 twist hobbit rifle has a stability factor of 1.64 in those conditions and would likely give much better performance.

 

[Bluefish]

Hornady talked about this a little in the eldx podcast. They said its almost the opposite. Slower twist will tumble easier than fast twist. When the developed the eldx to test at short range they spun them faster to duplicate the impact at long range up close.

 

[Marbles]

Hornady talked about this a little in the eldx podcast. They said its almost the opposite. Slower twist will tumble easier than fast twist. When the developed the eldx to test at short range they spun them faster to duplicate the impact at long range up close.

A tumbling bullet is unpredictable in tissue (which is more complex than gel) I would not use one for hunting. Though, if things go well they can certainly produce spectacular wound channels. Or, so is my take. I will track down te podcast and see if it changes my mind.

 

[Bluefish]

If I remember correctly they said that slowing down a bullet to test long range penetration is not accurate as spin and velocity do not decay at the same rate. Spin decays slower so a bullet that is shot at a lower velocity and thus slower spin can give the illusion of good performance while irl it will just pencil through as it’s more stable spinning faster and less velocity. To combat this they had a faster twist barrel made and then tested at short range and lower velocity to duplicate long distance impacts.
The exact opposite of the theory of the 8.6 bo. Ie spin it super fast to initiate opening. From the work maker has done, that theory does work for the subsonic solids. They have photos of tests with the petals opened and twisted from the spin.

 

[Marbles]

Ok, podcast confirms what I was thinking. RPM decays at a significantly slower rate then velocity, so a bullet that has a slower initial velocity will have a slower impact RPM and less stability.

The tumbling results in inconsistent performance (per the podcast) and consistency is more important to me than crossing my fingers and hoping for lady luck's blessing.

So, the question is, at the point of being stable enough for consistent performance, does a fast spin help over just fast enough?

 

[Marbles]

If I remember correctly they said that slowing down a bullet to test long range penetration is not accurate as spin and velocity do not decay at the same rate. Spin decays slower so a bullet that is shot at a lower velocity is less stable and thus can give the illusion of good performance while irl it will just pencil through as it’s more stable and less velocity. To combat this they had a faster twist barrel made and then tested at short range and lower velocity to duplicate long distance impacts.
The exact opposite of the theory of the 8.6 bo. Ie spin it super fast to initiate opening. From the work maker has done, that theory does work for the subsonic solids. They have photos of tests with the petals opened and twisted from the spin.

Sorry we posted at the same time. Yes, you remember correctly. Though they talk about tumbling being inconsistent.

 

[Bluefish]

Sorry we posted at the same time. Yes, you remember correctly. Though they talk about tumbling being inconsistent.

It probably is. When it’s right, it is deadly, when it’s not, poor performance. Also velocity sounded critical, which correlates with the 223 thread and my current thinking. Get the bullet to the target at the correct speed and it works. Faster spin rate is never bad until things come apart.

 

[Formidilosus]

I'm guessing a 178 gr ELD-X is a bad choice from my Tikka 308 with 1:11 twist given a sea level, 60 F stability factor of 1.35. The same bullet from my 1:10 twist hobbit rifle has a stability factor of 1.64 in those conditions and would likely give much better performance.

That’s not a bad choice at all. We have and do shoot 208gr AMAX/ELD-M’s and other of the ilk from 1-11” twist 308’s. Yes faster twist generally is better, however people have went too far on the other side where somehow a 1-11” twist isn’t enough for a 180gr bullet.


As for twist/RPM and terminal ballistics- it depends. In general if a bullet has sufficient velocity to initiate upset via expanding and especially fragmenting, then higher RPM helps in upset. However if the bullet does not have sufficient velocity remaining, then lower RPM can help destabilization earlier and yawing (tumbling).

 

[Marbles]

That’s not a bad choice at all. We have and do shoot 208gr AMAX/ELD-M’s and other of the ilk from 1-11” twist 308’s. Yes faster twist generally is better, however people have went too far on the other side where somehow a 1-11” twist isn’t enough for a 180gr bullet.


As for twist/RPM and terminal ballistics- it depends. In general if a bullet has sufficient velocity to initiate upset via expanding and especially fragmenting, then higher RPM helps in upset. However if the bullet does not have sufficient velocity remaining, then lower RPM can help destabilization earlier and yawing (tumbling).

Out of curiosity, because stability is something I have not thought of much before, is there a point prior to a stability factor of 1 where more stability is needed?

If I'm using a ballistics calculator and shoot in interior Alaska in winter, should there be a point where I should select a shorter bullet with more stability?

I.e. 178 gr ELD-X at 500 ft ASL in -40 has a stability factor of 1.09 if started out at 2450 fps (guestimate for a 16 inch barrel) at a barometric pressure of 30.56 inHg (upper end of winter average baro reading).

 

[Formidilosus]

Out of curiosity, because stability is something I have not thought of much before, is there a point prior to a stability factor of 1 where more stability is needed?

There is “stable” as in bullets stay nose first; then there is “stable” as in maximizing available BC. There are no conditions where a functional length 1-11” twist barrel isn’t going to stabilize (nose first) a 178gr ELD-X.

You are pole vaulting over mouse turds.

 

[KClark]

Julian Hatcher discussed his experiments with twist rates in Hatchers Notebook. IIRC he determined the 270 Win and 220 Swift would penetrate thicker steel than M2 AP black tip due to higher RPM and it was held as a secret until after WWII. How this relates to flesh and bone I'm not sure.

I no longer have a copy of his book or I'd look it up and be more specific.

 

[Marbles]

You are pole vaulting over mouse turds.

I'm good at that. Just don't let it get much bigger because I suck as a pole vaulter.

Thanks.

 

[Lou270]

I have studied this a bit. The rotational energy is very small to begin with and changing to faster twist increaes very little (think neighborhood of 10 ft-lbs) Ballisticians across the board dismiss twist as having any impact on terminal performance - assuming a bullet is stable to begin with and expands. There is just very little energy there. Internet experts say a lot of different things but nobody who studies ballisitcs and forensics and published books or papers thinks it does

As for tumbling inside game - no amount of rifling twist will keep a bullet stable in flesh. Flesh is roughly 1000x more dense than air so consider amount of spin increase needed to make stable

What keeps a bullet penetrating straight is shoulder stabilization. You can google this but there is a reason flat front solids track straight…

So what does faster twist do. In general 3 things:

Decrease angle of incidence. Any bullet tip is “wobbling” some when it leaves the barrel. After 80-100 yards this is pretty much gone. Faster twist can decrease this wobble some. Not much of an issue with soft points, tipped, big hollow points etc. as they are more forgiving to start expansion. However some bullets with long narrow hollow points may start to tumble at closer ranges before reaching terminal shape.

Also, for bullets with no means to expand (think otm/fmj) the more stable a bullet is going in the longer it will penetrate before tumbling. This is why there are mixed reviews on m16 coming out of viet nam. Sometimes tumbled and came apart and did a lot of damage. Sometimes penciled through as never tumbled. So less stability is your friend there.

The centrifugal force can cause a bullet to expand more quickly. By far the dominant force in expansion is dynamic pressure (ie from foward motion) but particularly with very thin jackets with nothing to control (jacket thickness/partition/bonding) this happens. I doubt a big factor in big game bullets unless maybe somebody uses match type tipped bullets or uses a way faster twist than bullet is designed for

This last one is more my opinon and never seen mentioned in literature. More spin for “petal bullets” can present more frontal area in shorter distance for bullet to transfer energy. Not a big deal with normal mushrooms but petaled bullets (ie barnes x, etc) have space between them. They penetrate deep because there is less surface area crushing tissue as they penetrate. Spinning them faster presents more frontal area in the same distance. Of course that will decrease penetration some. Think of the swirlies (which by the way are caused by material flowing off tips of petals) in the gel videos. Faster twist should show more swirls and possibly wider but shorter wound

People say there is “cutting” going on with petaled bullets but I highly doubt that. #1 the temp cavity forms as the bullets is passing so the sides of the bullet are not touching anything until the bullet slows down. This is why even in media you see a bullet spin. Much like air this is because the bubble forms around it and nothing to degrade spin Once the bullet slows down and media touches the sides of bullet near end of peneration. the rotations stop pretty quick - since there is very little energy there

Lou

 

[Marbles]

Thank you for taking the time to write a long and detailed reply.

I have studied this a bit. The rotational energy is very small to begin with and changing to faster twist increaes very little (think neighborhood of 10 ft-lbs) Ballisticians across the board dismiss twist as having any impact on terminal performance - assuming a bullet is stable to begin with and expands. There is just very little energy there. Internet experts say a lot of different things but nobody who studies ballisitcs and forensics and published books or papers thinks it does

It is not a lot compared to the KE from forward motion, but if I got the math correct for a 178 gr 30 cal cylinder (yes I know, a bullet is not a cylinder) it comes out to 34.8 ft.lb for 1:11 twist and 52.0 ft.lb for 1:9 twist. Enough KE to put an arrow through a medium bodied animal, which is not insignificant.

The question really is how does it effect how energy is transfer, because it is only transferred energy that destroys tissue, why numbers alone are pretty meaningless.

My impression is it simply has not been tested or considered. Considering the garbage I see spouted be ammunition companies, I only trust a ballistician if they bring data.

As for tumbling inside game - no amount of rifling twist will keep a bullet stable in flesh. Flesh is roughly 1000x more dense than air so consider amount of spin increase needed to make stable

This contradicts what Hornady's ballistician says on podcast 087, where they report twist specifically effected stability in gel.

What keeps a bullet penetrating straight is shoulder stabilization. You can google this but there is a reason flat front solids track straight…

So what does faster twist do. In general 3 things:

Decrease angle of incidence. Any bullet tip is “wobbling” some when it leaves the barrel. After 80-100 yards this is pretty much gone. Faster twist can decrease this wobble some. Not much of an issue with soft points, tipped, big hollow points etc. as they are more forgiving to start expansion. However some bullets with long narrow hollow points may start to tumble at closer ranges before reaching terminal shape.

True, but at about 4 yards the behavior of a bullet against tempered glass becomes predictable, materially insignificant at much closer ranges than 80 yards (see The Effects of Commercial Tempered Glass on Rifle Bullets). This would support your position that a faster twist is insignificant. My bet is that within 4 yards, rather than yaw it is the effect of muzzle blast that makes things unpredictable.

Also, for bullets with no means to expand (think otm/fmj) the more stable a bullet is going in the longer it will penetrate before tumbling. This is why there are mixed reviews on m16 coming out of viet nam. Sometimes tumbled and came apart and did a lot of damage. Sometimes penciled through as never tumbled. So less stability is your friend there.

I don't shoot non-expanding bullets from rifles for a reason. So, I don't see this as applicable.

The centrifugal force can cause a bullet to expand more quickly. By far the dominant force in expansion is dynamic pressure (ie from foward motion) but particularly with very thin jackets with nothing to control (jacket thickness/partition/bonding) this happens. I doubt a big factor in big game bullets unless maybe somebody uses match type tipped bullets or uses a way faster twist than bullet is designed for

I would like to see the effect quantified. I bet in rifle bullets it is large enough to be important, but not sure how important.

This last one is more my opinon and never seen mentioned in literature. More spin for “petal bullets” can present more frontal area in shorter distance for bullet to transfer energy. Not a big deal with normal mushrooms but petaled bullets (ie barnes x, etc) have space between them. They penetrate deep because there is less surface area crushing tissue as they penetrate. Spinning them faster presents more frontal area in the same distance. Of course that will decrease penetration some. Think of the swirlies (which by the way are caused by material flowing off tips of petals) in the gel videos. Faster twist should show more swirls and possibly wider but shorter wound

I don't think that plays out, the bullet is not spinning that fast in relation to forward motion, even if spin rate doubled in relation to forward velocity, if the bullet was making 1 rotation for ever 8 inches initially, it would only be making 1 rotation for every 4 inches of travel after impact so frontal area would not behave like it was larger.

As the petals expand, conservation of rotational inertia will result in a rapid decrease in angular velocity (slowing of rotation, think of a ballet dancer speeding up as they pull their arms in and slowing as they extend them). Consequently, significant resistance is not needed to slow rotation of an expanding bullet on impact. Though the swirl than can be seen in ballistics gell suggest rotational energy is transfered to the target.

People say there is “cutting” going on with petaled bullets but I highly doubt that. #1 the temp cavity forms as the bullets is passing so the sides of the bullet are not touching anything until the bullet slows down. This is why even in media you see a bullet spin. Much like air this is because the bubble forms around it and nothing to degrade spin Once the bullet slows down and media touches the sides of bullet near end of peneration. the rotations stop pretty quick - since there is very little energy there

Lou

I agree. It depends on how the word cut is used, but other that Winchester Ranger T bullets I doubt cutting in the sense of a knife or broadhead has any notable role in terminal effects. Even with the Ranger Ts the effect only increases the odds of damaging a major blood vessel by like 6% (or so I read years ago). In rifle bullets at normal velocity such an effect would be even smaller given that temporary cavity is significantly more important.

 

[Swampcruiser]

I am shooting the 178 grain ELD-x out of my Tikka 1:11 damn near at sea level unfortunately, at 80 degrees and at 400 yards it's clean holes in the target

 

[Lou270]

Thank you for taking the time to write a long and detailed reply.

It is not a lot compared to the KE from forward motion, but if I got the math correct for a 178 gr 30 cal cylinder (yes I know, a bullet is not a cylinder) it comes out to 34.8 ft.lb for 1:11 twist and 52.0 ft.lb for 1:9 twist. Enough KE to put an arrow through a medium bodied animal, which is not insignificant.

The question really is how does it effect how energy is transfer, because it is only transferred energy that destroys tissue, why numbers alone are pretty meaningless.

My impression is it simply has not been tested or considered. Considering the garbage I see spouted be ammunition companies, I only trust a ballistician if they bring data.

This contradicts what Hornady's ballistician says on podcast 087, where they report twist specifically effected stability in gel.

True, but at about 4 yards the behavior of a bullet against tempered glass becomes predictable, materially insignificant at much closer ranges than 80 yards (see The Effects of Commercial Tempered Glass on Rifle Bullets). This would support your position that a faster twist is insignificant. My bet is that within 4 yards, rather than yaw it is the effect of muzzle blast that makes things unpredictable.

I don't shoot non-expanding bullets from rifles for a reason. So, I don't see this as applicable.

I would like to see the effect quantified. I bet in rifle bullets it is large enough to be important, but not sure how important.

I don't think that plays out, the bullet is not spinning that fast in relation to forward motion, even if spin rate doubled in relation to forward velocity, if the bullet was making 1 rotation for ever 8 inches initially, it would only be making 1 rotation for every 4 inches of travel after impact so frontal area would not behave like it was larger.

As the petals expand, conservation of rotational inertia will result in a rapid decrease in angular velocity (slowing of rotation, think of a ballet dancer speeding up as they pull their arms in and slowing as they extend them). Consequently, significant resistance is not needed to slow rotation of an expanding bullet on impact. Though the swirl than can be seen in ballistics gell suggest rotational energy is transfered to the target.

I agree. It depends on how the word cut is used, but other that Winchester Ranger T bullets I doubt cutting in the sense of a knife or broadhead has any notable role in terminal effects. Even with the Ranger Ts the effect only increases the odds of damaging a major blood vessel by like 6% (or so I read years ago). In rifle bullets at normal velocity such an effect would be even smaller given that temporary cavity is significantly more important.p

As for energy that 10–20 ft-lbs is about the equivalent of a 177 cal pellet and insignificant compated to 1000+ from forward motion. As for the arrow comparison that is apples to oranges and a terribly misued analogy (even by people who know better). An arrow only going 1-150 fps kills fine. I guess velocity doesn’t matter either. Not sure why people only use that analogy for energy…

Bullet performance depends on what the bullet is designed for. You take a 22 cal bullet designed for a 1-14 twist and shoot it in a 22 cal with 1-7 twist probably behave differently. Take a big game bullet designed for a 1-10 twist cartridge and shoot in faster twist may see some difference though doubt would notice it.

If don’t understand it, google shoulder stabilzation. The density change from elevation can make a bullet unstable in air let alone flesh. You can keep a bullet from tumbling slightly longer (few inches) at closer ranges by decreasing angle at of incidence but bullets get more stable as they go down range. I would be disappointed if hornady states something different as this has only been known about 100 years

Lou

 

[ElPollo]

A tumbling bullet is unpredictable in tissue (which is more complex than gel) I would not use one for hunting. Though, if things go well they can certainly produce spectacular wound channels. Or, so is my take. I will track down te podcast and see if it changes my mind.

So does that mean the vaunted 215 Berger or other similar bullets should not be used for hunting? Not trying to be ass but legitimately asking. By unpredictable do you mean a straight flight path or reliable fragmentation? I am not defending Bergers and have not used them. The only similar bullet I have experience with is the Lapua Scenar. In that experience (3 deer), all of them tumbled, which is how they upset. The wound channels didn’t open up until 4-5” in but were pretty consistent in a small sample size of 3. They also did not seem to deviate hugely from the shot path. In my experience, they were predictable, but didn’t perform the way I wanted.

 

[ElPollo]

After having read further into the thread, it seems you are referring to stability in terms of deviation from the shot path. If you are talking bullets designed to tumble in order to initiate fragmentation, my limited experience and what I’ve seen in hell tests performed by others suggests they deviate less than for bullets that tumble and don’t upset. Watch some of Simple Minded Fella’s gel tests on YouTube. While I don’t always agree with all of his conclusions, I like that he uses real organic ballistic gelatin to look for lower limits of upset velocity across a lot of bullets. The bullets that tumble but don’t upset often exit the sides of the block. The ones tumble and then upset tend to stay in the blocks and are fairly straight. The variability that results in decreased predictability for fragmenting bullets is impact speed. Bullets that are designed to tumble but not fragment, i.e. FMJs, are less predictable in terms of shot path deviations across a range of velocities.

 

[Marbles]

So does that mean the vaunted 215 Berger or other similar bullets should not be used for hunting? Not trying to be ass but legitimately asking. By unpredictable do you mean a straight flight path or reliable fragmentation? I am not defending Bergers and have not used them. The only similar bullet I have experience with is the Lapua Scenar. In that experience (3 deer), all of them tumbled, which is how they upset. The wound channels didn’t open up until 4-5” in but were pretty consistent in a small sample size of 3. They also did not seem to deviate hugely from the shot path. In my experience, they were predictable, but didn’t perform the way I wanted.

If a bullet reliably tumbles and fragments every time, and creates consistent wound channels, I see no problem with it. I would want a large sample and I doubt this is the case, even with Bergers.

I forget the thread, perhaps the 223 thread, but it is discussed how the nose on Bergers reliably collapses, which is why the are consistent. Other match BTHP bullets have a different tip construction and depend on tumbling, which is why they give inconsistent performance.

Notice, not a single recommend bullet in the 223 thread depends on tumbling to initiate fragmentation.