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Thanks JCow. I wish I understood terminal ballistics better. I was simply questioning this on aspect. Thanks for all the replies!
Effect of Bullet Spin on Terminal Performance
- Thread starter Tiny Tim
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IMO, the rate of spin alone given an un-damaged or weakened jacket would have little difference in terminal effect in game. Differences in terminal ballistics can be caused by twist rate when it effects the bullets jacket integrity and the RPM's approach or exceed the "blow-up" threshold. This is commonly seen when shooting lightly constructed, frangible varmint bullets at high velocity and fast twist rates.
This is a formula determining twist rate RPM's. The RPM's for bullet core separation/blow-up which occurs at 310,000 to 330,000 RPM's.
(Muzzle velocity) x (720) /twist-rate =RPM
Blasting a 6.5mm lead core bullet at 3300FPS with a fast twist(7.5), would result in 317,000 RPM's ...potentially a recipe for erratic bullet performance.
I think it is quite possible that a bullet that is near, or at he threshold will be in weakened state and show a difference in terminal effect on game .....if it doesn't blow up before it even reaches the animal....IMO
This is a formula determining twist rate RPM's. The RPM's for bullet core separation/blow-up which occurs at 310,000 to 330,000 RPM's.
(Muzzle velocity) x (720) /twist-rate =RPM
Blasting a 6.5mm lead core bullet at 3300FPS with a fast twist(7.5), would result in 317,000 RPM's ...potentially a recipe for erratic bullet performance.
I think it is quite possible that a bullet that is near, or at he threshold will be in weakened state and show a difference in terminal effect on game .....if it doesn't blow up before it even reaches the animal....IMO
cohunt
Well-Known Member
A few years ago i read an online documentation where a guy did a test of twist rate vs bullet expansion.
He did a test of both lead core hunting bullets and monolithic hunting bullets (no target bullets were tested) he had 2 rifles in 6.8spc, 1 was a 1:12 twist and the other was a 1:8.
He loaded same charges with same bullets and shot them for expansion at 100,200&300 yards.
At all distances the bullets in the1:8 expanded better than the 1:12, and in some cases the 1:12 veered off course where the 1:8's did not. The additional expansion was even more pronounced in the monolithic bullets than the lead core bullets, but he used an attached petal monolithic bullet rather than a detaching petal design.
Wish I knew where to find that test, but it has lapsed from my memory in the last few years
As a side note, he also did a "reduced" velocity vs distance test to see if a bullet fired at a reduced velocity at close ranges matched the same velocity at longer distances and he noted that the bullets fired at full velocity - their expansion at distance of 400, vs a bullet fired at 50 yards at reduced velocity did not exhibit the exact same expansion-- he surmised that this was die to faster spin at distances due to the increased muzzle velocity.
He did a test of both lead core hunting bullets and monolithic hunting bullets (no target bullets were tested) he had 2 rifles in 6.8spc, 1 was a 1:12 twist and the other was a 1:8.
He loaded same charges with same bullets and shot them for expansion at 100,200&300 yards.
At all distances the bullets in the1:8 expanded better than the 1:12, and in some cases the 1:12 veered off course where the 1:8's did not. The additional expansion was even more pronounced in the monolithic bullets than the lead core bullets, but he used an attached petal monolithic bullet rather than a detaching petal design.
Wish I knew where to find that test, but it has lapsed from my memory in the last few years
As a side note, he also did a "reduced" velocity vs distance test to see if a bullet fired at a reduced velocity at close ranges matched the same velocity at longer distances and he noted that the bullets fired at full velocity - their expansion at distance of 400, vs a bullet fired at 50 yards at reduced velocity did not exhibit the exact same expansion-- he surmised that this was die to faster spin at distances due to the increased muzzle velocity.
Asco6
Member
Thanks Cohunt, that's a bit of info I didn't know. I love it when actual science/ data get posted on this forum. I will keep that info and calculate it in to my rifle/ barrel selections.
I was wondering if there was really any advantage to my 1:8 HCA 7mm barrel over the standard 1:9.5, but the above info is heartening, I will likely get slightly better terminal performance with my hunting bullets.
Real info is a nice breake from the "filler" posts on this forum like:
"Throat erosion! You'll burn out your barrel; try my favourite Mashburn; I hate Creedmoores"
I was wondering if there was really any advantage to my 1:8 HCA 7mm barrel over the standard 1:9.5, but the above info is heartening, I will likely get slightly better terminal performance with my hunting bullets.
Real info is a nice breake from the "filler" posts on this forum like:
"Throat erosion! You'll burn out your barrel; try my favourite Mashburn; I hate Creedmoores"
I've done a fair bit if bullet testing and based on what I've seen, greater stability (a function of RPMs) has a significant impact on terminal performance. With spitzer style bullets it seems like roughly 190,000 RPMs on the low end is the magic number to get consistent and predictable expansion and penetration. This is dependent on bullet design though.
Large bore dangerous game bullets function well at lower RPMs but there's still a strong following for using fast twist barrels on DG rifles to provide straighter penetration with solids.
Large bore dangerous game bullets function well at lower RPMs but there's still a strong following for using fast twist barrels on DG rifles to provide straighter penetration with solids.
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Stgraves260
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I know there are people out there that calculate spin drift. I'm just not one of them. I do know as your Velocity changes so will the rate of the spin drift . It's really hard to calculate spin drift when you are messing with wind. So unless there is a way out there to eliminate ALL wind while taking that 2000 yards shot, how can you tell what the twist rate of the barrel dos to the bullet? You can't. It's just all Theory.
The terminal performance of a bullet is definitely affected be the spin rate.
Let's use the a 28 Nosler/195 EOL hybrid for an example.:
When Berger designed the 195 EOL it was designed for use on large deer species. Ideally, it needs an 8 twist to stabilize. The twist rate (rotational speed) also effects the time after initial penetration that the bullet fragments. Faster twist will fragment sooner and slower twists a little later. ( centrifugal force ) You can't expect a 195 to work on an antelope or coyote the same way as on an elk or moose.... IMO - That is what the 150-168 gr. bullets are for. Problem is that an old school 150 gr. bullet probably won't shoot very well out of 28 Nosler set up with an optimum freebore for the 195. ( unless you use a copper hunting bullet like the Hammers, CE, Barnes etc...)
Many complaints of poor terminal performance for any bullet usually are a from the mindset that one bullet can do it all and poor judgment of shot placement.
What you have to remember is what type of bullet you plan to shoot with and is it applicable to the size of game being hunted and finally at what range will most of hunting be at... Don't try to second guess the terminal effects of spin rate, The manufactures already did it for you long before they released the bullet for sale to the public.
Berger bullets are designed to fragment apart and disperse their energy in the chest cavity over a large area. A Nosler or Swift are designed to expand in the classic mushroom shape and spread their energy over a smaller area with possibly less meat damage. A Barnes mono bullet expands much like the classic partition and holds together at much higher velocities but the trade off is at lower speeds they don't expand well. Remember too that full copper bullets also expand at a slower rate when going thru game and will generally have better penetration but sacrifice that wide damage path like the Berger gives.
Those big DG African cartridges are made of brass, They are old designs that came about because lead core bullet designs of that time couldn't get the job done. A 30 cal lead bullet expands to .5" cal and a .375 expands to .6". So they designed the solids to be the expanded size of lead core and then kept adding length to get the penetration they needed to drop the type of DG game hunted. So the spin rate fore these are to stabilize the bullet but have no effect on terminal performance...
Let's use the a 28 Nosler/195 EOL hybrid for an example.:
When Berger designed the 195 EOL it was designed for use on large deer species. Ideally, it needs an 8 twist to stabilize. The twist rate (rotational speed) also effects the time after initial penetration that the bullet fragments. Faster twist will fragment sooner and slower twists a little later. ( centrifugal force ) You can't expect a 195 to work on an antelope or coyote the same way as on an elk or moose.... IMO - That is what the 150-168 gr. bullets are for. Problem is that an old school 150 gr. bullet probably won't shoot very well out of 28 Nosler set up with an optimum freebore for the 195. ( unless you use a copper hunting bullet like the Hammers, CE, Barnes etc...)
Many complaints of poor terminal performance for any bullet usually are a from the mindset that one bullet can do it all and poor judgment of shot placement.
What you have to remember is what type of bullet you plan to shoot with and is it applicable to the size of game being hunted and finally at what range will most of hunting be at... Don't try to second guess the terminal effects of spin rate, The manufactures already did it for you long before they released the bullet for sale to the public.
Berger bullets are designed to fragment apart and disperse their energy in the chest cavity over a large area. A Nosler or Swift are designed to expand in the classic mushroom shape and spread their energy over a smaller area with possibly less meat damage. A Barnes mono bullet expands much like the classic partition and holds together at much higher velocities but the trade off is at lower speeds they don't expand well. Remember too that full copper bullets also expand at a slower rate when going thru game and will generally have better penetration but sacrifice that wide damage path like the Berger gives.
Those big DG African cartridges are made of brass, They are old designs that came about because lead core bullet designs of that time couldn't get the job done. A 30 cal lead bullet expands to .5" cal and a .375 expands to .6". So they designed the solids to be the expanded size of lead core and then kept adding length to get the penetration they needed to drop the type of DG game hunted. So the spin rate fore these are to stabilize the bullet but have no effect on terminal performance...
I don't know if this is even relevant, but some of the new "fluted" handgun loads don't use bullet expansion at all, they rely on something like hydraulic shock to inhance performance. By my logic, spin would seem to be a big contributor to their effect. I haven't used any, but friends who have report impressive results even at handgun velocities.
I'll chime in here without getting too physics-y, on two counts: first, the force involved, and second, the rotational decay.
In the first case, the bullet is held together (stopped from flying apart) by the material strength of the bullet - lead, jacket, copper, whatever. For a given caliber, the force required to hold a bullet together increases as the square of the rotational speed - a bullet rotating at 40% greater rate than another requires twice the force. That outward force bears on the expansion rate of the bullet as it deforms and "mushrooms." Higher spin, more rapid or greater total mushrooming, all other things held equal. Conversely, the more rapid the mushrooming, the greater the braking force on the rotation rate as tissue drags on the increasing diameter of the bullet. So the wound channel profile differs for different spin rates.
Second, while it is true that a bullet is decaying in spin rate in air slowly, and its maximum rotation rate appears to be that of the barrel twist times the velocity, you can see that the ratio of rotation to velocity increases with distance up to the point of contact with the animal, in effect increasing in spin rate as measured in rotations per linear foot; a 1:10 twist might be a 1:8 at impact. At the point of contact, the bullet decelerates dramatically - but the rotational rate does not. Accordingly, while a bullet leaving the muzzle might spin one time in 10 inches, inside an animal that would change, because of the rapid linear deceleration compared to the slow rotational rate deceleration, to a full rotation in perhaps 2 or 3 inches, with mushrooming affecting the rate of decrease of rotational speed.
If the bullet opens like a Black Talon, then, the aggregate tissue damage from the "buzzsaw" effect would be far more pronounced at a higher rotational speed. You can see this from the wound cavity geometry in clear gel. The spiral track of the cavity might show a half-dozen spins within a distance of 16 inches, yet no barrel has a twist rate of one in 3 inches.
The net: higher destructive force with a greater spin rate, all else held equal; and expansion brings a measure of inequality. The above does not account for tumbling - only for stable transit of a bullet.
In the first case, the bullet is held together (stopped from flying apart) by the material strength of the bullet - lead, jacket, copper, whatever. For a given caliber, the force required to hold a bullet together increases as the square of the rotational speed - a bullet rotating at 40% greater rate than another requires twice the force. That outward force bears on the expansion rate of the bullet as it deforms and "mushrooms." Higher spin, more rapid or greater total mushrooming, all other things held equal. Conversely, the more rapid the mushrooming, the greater the braking force on the rotation rate as tissue drags on the increasing diameter of the bullet. So the wound channel profile differs for different spin rates.
Second, while it is true that a bullet is decaying in spin rate in air slowly, and its maximum rotation rate appears to be that of the barrel twist times the velocity, you can see that the ratio of rotation to velocity increases with distance up to the point of contact with the animal, in effect increasing in spin rate as measured in rotations per linear foot; a 1:10 twist might be a 1:8 at impact. At the point of contact, the bullet decelerates dramatically - but the rotational rate does not. Accordingly, while a bullet leaving the muzzle might spin one time in 10 inches, inside an animal that would change, because of the rapid linear deceleration compared to the slow rotational rate deceleration, to a full rotation in perhaps 2 or 3 inches, with mushrooming affecting the rate of decrease of rotational speed.
If the bullet opens like a Black Talon, then, the aggregate tissue damage from the "buzzsaw" effect would be far more pronounced at a higher rotational speed. You can see this from the wound cavity geometry in clear gel. The spiral track of the cavity might show a half-dozen spins within a distance of 16 inches, yet no barrel has a twist rate of one in 3 inches.
The net: higher destructive force with a greater spin rate, all else held equal; and expansion brings a measure of inequality. The above does not account for tumbling - only for stable transit of a bullet.
GMFWOODCHUCK
Member
This is all stuff that's been tested on there Aberdeen Testing grounds. It's not theory. There's a strong science behind it. The army using non spinning flachettes in it's smoothbore tank guns isn't done on accident. Less rotations = less expansion = more armor penetration. Their 6 inch bolts aren't capable of penetrating 6 meters (yes, 6 meters) of hardened steel because someone happen chanced it. It was carefully planned and designed.
That said, the science isn't understood by most of us. Most of us aren't dynamic engineers and therefore aren't going to know the math. And nor is it needed to be known. All you need to know is that the faster the rotations the faster the expansion. There's a reason why they tell you not to use 22 hornet bullets in 22-250s with a fast twist. They come apart in flight before going very far. So obviously that will have an effect on a woodchuck versus one spinning quite slowly.
The easiest way to understand it is with a kid's ride. The faster you spin the harder it is to hang on.
That said, the science isn't understood by most of us. Most of us aren't dynamic engineers and therefore aren't going to know the math. And nor is it needed to be known. All you need to know is that the faster the rotations the faster the expansion. There's a reason why they tell you not to use 22 hornet bullets in 22-250s with a fast twist. They come apart in flight before going very far. So obviously that will have an effect on a woodchuck versus one spinning quite slowly.
The easiest way to understand it is with a kid's ride. The faster you spin the harder it is to hang on.
Don't have the time to get into the math right now, but you can figure out how many ft. Lbs. of energy there is in the rotation. While it's only a few percent of the forward energy, it still has an effect. Faster twist in my experience have more rapid bullet upset terminally.
Exactly!!
One exception is the your example of the buzzsaw affect of the old black talon.
No doubt its design does affect terminal performance but you also have to consider that its the hydrostatic shock (energy) that has a greater effect on tissue damage.
These factors of terminal performance are designed into the bullet by the manufacturer. The manufacturers are usually very specific on twist rate and minimal/maximum speed that the bullet is designed for.
So say you pick a twist rate that like a 1:7.5 for a 6.5 Creedmore shooting 143-156 gr bullets @ 2700-2800fps. Would you pick the same 1:7.5 twist for a 6.5-300 Wby solely based on the idea that it will be more terminal affect? I wouldn't... because that 143 will probably self destruct as soon a it leaves the barrel.
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Thanks greyfox. Each bullet structure will have it's own amount of rotational force it can withstand before self destructing. I also assume that different rifling profiles may also contribute as some are "harder" on bullets than others.
Thanks to you cohunt for the post on actual scientific data.
And thanks to you GeorgeS. While I realized that the spin decay rate was significantly less than the velocity decay rate which gives greater stability, I had not put together that its net affect was similar to an increasing twist rate as distance increases.
Thanks to you cohunt for the post on actual scientific data.
And thanks to you GeorgeS. While I realized that the spin decay rate was significantly less than the velocity decay rate which gives greater stability, I had not put together that its net affect was similar to an increasing twist rate as distance increases.
mwkelso
Well-Known Member
This calculation is assuming a 1:12" twist...
For quite a while I could not figure out where "720" came from. Turns out it's simply 12*60, or 12 bullet rotations per 60 seconds for a twist velocity in rpm.
Here is an article from accurateshooter.com that breaks down the theory behind twist velocity, and also goes a little deeper into the calcs behind it.
https://www.accurateshooter.com/technical-articles/calculating-bullet-rpm-spin-rates-stability/
To the OP:
If your looking for optimal spin, you might consider looking at either Greenhill's or Miller's optimization equations. They have helped settle a lot of the controversy for me anyway. Not to disregard any of the above comments and suggestions. There is definitely a lot of wisdom in there, far beyond what I have in experience.
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