Bullet impact and twist rate

sleddogg

sleddogg

New member
Is there a difference in bullet performance on game animals relating to twist?

Lets use for example a 223 Remington shooting a 55 grain Nosler BT at 2900 FPS. One shooter uses a 1 in 12 twist, one is shooting a 1 in 7 twist. Any difference in impact when both projectiles hit a coyote?
 
Are you talking location of impact (POI) or are you talking about how the bullet reacts upon impact due to the different twist rates?

I am currious to see what some of the replies to your question are.
 
Crapshoot - I'm talking about bullet performance, expansion, penetration, knockdown power, etc.

All theories invited, as I don't have a clue, and curious if there are any studies done on this aspect of terminal ballistics.
 
IIRC (a big if), I read an article stating that thin jacketed projectiles would expand more radically when run through fast twist tubes. BUT it was very specific about the results being from FAST calibers, with HEAVY twist tubes, using LIGHT bullets. After that, I beleive it was inconclusive.

The heavy twist always gives you the best range of bullet use however, so the faster the twist, the more versatile the rifle.
 
When I used to use an old 722 Remington in 222 exclusively for coyotes (don't know the twist rate offhand) and compare that to the 223 with a 1"/9" with the same bullets but about 300 fps faster at normal shooting distances I really cannot tell the difference between the two on the coyotes. I don't know if that answered your question to a tee but I hope it helps.
Sleddog, the velocity you quoted is pretty slow for a 223. That is more inline with a 222 loading.
 
With all else being equal, a faster twist "might" produce more velocity however, since Sundays testing of various barrel lengths and twist I don't think you will be able to establish any repeatable differences. If you will look at my results you will see the slowest twist in the 20" barrel produced the fastest velocity while a fast twist in the 16" barrel was fastest. Like I said velocity relies more on internal barrel dimensions/quality than what twist or length. If anyone will try the same test with a chronograph and different lengths/twists I would be very interested in your results. Me and my buddy who supplied 3 of the guns and the chronograph are going to do more. I saw several theories blown out of the water.

As for killing difference... I have never noticed any related to twist but it is an interesting theory.
 
A lightly constructed bullet pushed at a fast enough velocity and with a high enough rpm (fast twist) can be made to disintegrate without hitting anything at all. Those same (almost) parameters will cause it to come apart if it hits even the smallest twig.

You can easily verify it for yourself. Load a lightly constructed bullet up as fast as you safely can (4500+ fps 40gr) in your fast twist 22-250 and shoot one shot through 2 sheets of cardboard separated by a foot or so.

The first sheet will have a nice .224 hole in it, the second will have a million tiny little spatter holes (and maybe a couple of larger holes). That same round loaded at 3200 fps will put a .224 hole in both sheets. I figured this one out first by accident and then did some informal "testing".

Obviously the answer to your original question is yes twist rate can affect terminal ballistics IF (great big "if") you push a relatively fragile bullet fast enough that it is on the ragged edge of coming apart anyway due to the centrifugal forces caused by high rpm.

Will it have an affect in the real world? Probably not, and I'm sure not at 2900fps.

If you have a fast twist barrel you probably got it to shoot longer (heavier) bullets anyway and you won't be able to push them to the required velocities.

By the way all else being equal a fast twist should give (slightly) slower velocities (and a slightly shorter barrel life) due to the increased friction.
 
There are many proponents of the .17 Remington who believe that its unexpected killing power is a direct result of both high muzzle velocity and extremely fast rpm's. Bullet rpms's are determined by a combination of velocity and barrel twist rates, and with its 1:9 or 1:10 barrel rifling, those .172" bullets are not only moving forward very fast, but they are also spinning longitudinally (perpendicular to their axis of flight) very fast. Seems like I remember reading somewhere that bullet rpm's regularly exceed 100,000 rpm's. When my 30 gr. .17 Rem bullet strikes a coyote, I suspect that a lot of its energy is transferred to the coyote via this rotational energy. If you've seen the famous slow-motion bobcat footage on the Coyote Gods videotape, that cat puffs up in all directions as the bullet energy is dumped inside. Makes sense to a country boy like me.
 
Quote:
Is there a difference in bullet performance on game animals relating to twist?

Lets use for example a 223 Remington shooting a 55 grain Nosler BT at 2900 FPS. One shooter uses a 1 in 12 twist, one is shooting a 1 in 7 twist. Any difference in impact when both projectiles hit a coyote?



There is a major difference. The faster twist makes the bullet expand much faster. I have been out shooting colony varmints with my 9" twist 223s alongside someone shooting a 12" twist 223 with the same bullets and you would think we were shooting different calibers. My hits are more explosive at 200 yards than his are at 25.
My MV was lower than his because I was shooting a mild load compared to his max load.

Jack
 
Jack what were the velocities?

Your hits were more explosive at 200 yards than his at 25 yards? Got video?
 
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If the bullet stays together to the target, more rpms= more flung goo, assuming all else is equal. This applies to small critters hit with fast projectiles.
Leon nailed it: faster twist, more friction...
F1
 
Below is a simple formula for determining the foot pounds of energy of a bullet at a given velocity.

Energy = Weight times Velocity Squared divided by 450,395 (fixed constant).

Example: 55 gr. bullet @ 3100 fps

55 x 3100 x 3100 divided by 450,395 = 1,173.53 foot pounds of energy
 
Took some Hornady 35gr V-Max bullets and shoved them as fast as I could in a 1-12" twist .22-250 barrel. Why? To see if I could get them to vaporize in mid-air. Yep, they sure did.
NMLeon seems to have hit everything square on the nail head about twist, velocity, and bullet jackets (and I know he can shoot).
 
Here's some interesting comparisons that are ultimately meaningless without knowing the engineering factors in a particular bullet's construction (and a lot of metallurgy) but are fun (and impressive) anyway.

In engineering terms it's the centripetal force (holding it together) not the centrifugal force (pulling it apart) that's measured, but centrifugal force is good shorthand and what is commonly understood so that's what I'll use.

The centrifugal forces on a bullet vary depending on the diameter/radius and the rate of spin. Think of riding a merry-go-round. The faster it spins and the closer you are to the edge, the more it wants to throw you off.

That's happening inside a bullet too. The centrifugal force trying to pull a bullet apart is zero along the center line (axis) of the spin and increases dramatically as you get close to the outside edge. The faster the velocity is, the faster the RPMs are (in a given barrel twist), and the more centrifugal force is trying to tear the jacket off the bullet.

The calculation to figure the Relative Centrifugal Force (RCF) of a spinning object is .00001118 X radius (in cm) X RPM squared = RCF (g).

This gives you just the g forces at the outside edge of a rotating object. Larger caliber bullets will have more RCF (with the same RPM) because of the greater distance from the axis to the outside edge, which is why smaller caliber bullets can (in general) be spun at faster rates of twist (RPMs).

For a .17 Rem at 4040fps out of a 1/9 twist barrel you come up with:

1.333 revolutions per ft (12" in a ft/the 9" it takes for one rev) X 4040 fps = 5386 revs per sec X 60 (secs in a minute)= 323,191.92 RPM square that and you get 104453017153.2864

The radius of a .17cal is .086" (half the diameter of a .172" bullet) X 2.54 (covert to cm) = .21844cm

.00001118 X .21844 X 104453017153.2864 = 255,090g

That's right! More than a quarter million gees (test pilots start blacking out at somewhere around 9g).

Plugging in the numbers for my 1/8 twist 22-250 at 4500fps (shooting 40gr but the barrel was made for up to 80gr pills), you come up with 405,000 RPM and an RCF of 521,679g. More than 1/2 million gees! I'm sure that bullet would have come apart on it's own if it had had just a little more time in flight for the gees to act on it.

Out of a (more normal) 1/12 twist .224 cal (at 4500fps) you "only" get 231,857 gees.

As far as a bullet blowing up inside an animal (from spin/velocity) you'd have to know all sorts of engineering data, or in the real world, just use trial and error. It would also depend on how far down range the impact was (remaining velocity determines RPM/RCF/etc.), and a whole host of other factors.

Sorry if I've bored you all to tears, as I said at the beginning, all this is fascinating (to me at least) but meaningless in real terms unless maybe you are a bullet designer.
 
Mods, please change nmleon's name to mrnmleon.

I'm anything but bored to tears, I just wish I had the mental capacity to lay things out like that. You stated that a faster twist should produce slower velocity because of friction while others say friction equals more pressure creating more velocity. I still say the differences are too slight to notice and the quality of a barrel will control the outcome but can you explain what would happen if quality was the same between 2 different barrels of different twist rates?
 
Tom-Actually measuring the real world affect of twist on velocity would be so difficult (and expensive) that I doubt anyone will ever try it in a rigid scientific/engineering test environment. Getting everything (except twist) absolutely identical in a series of barrels/bullets/conditions for testing would be a major engineering challenge in itself.

That said, the calculations are pretty straightforward to figure the energy needed needed to impart any given motion (spin in this case) to an object of any given mass.

The energy source is the same for spin and velocity. Faster spin takes more energy than slow spin leaving less energy remaining to impart linear (velocity) motion.

Among other factors, it is possible that the pressures (energy) may be increased in a specific barrel in the real world, or possible that (non twist related) friction will be decreased since your fast twist barrel is likely a custom match barrel. In either case the difference in total energy requirements might be greater than the increased energy needed to impart a faster twist and you could get better velocities out of your new barrel.

The best that can be done from from an engineering calculation point of view is to limit the variables to just the one you are calculating for. That's why we say all other factors being equal, in this case that would include pressure and friction.

Soo...(LOL), all else being equal faster twist barrels will result in lower velocities. Real world results may vary.
 
Leon, thank you sir for at least trying. I set out this past Sunday to establish some "truths" to some myths I guess you'd say and only came away more confused I guess. If you're bored take a look at the post 16" ar's.
 
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There is a major difference. The faster twist makes the bullet expand much faster. I have been out shooting colony varmints with my 9" twist 223s alongside someone shooting a 12" twist 223 with the same bullets and you would think we were shooting different calibers. My hits are more explosive at 200 yards than his are at 25.
My MV was lower than his because I was shooting a mild load compared to his max load.

Jack



Thats good to hear, as I've been baffled by results folks are getting from the .223. I hear lots of bang-flop stories at outrageous distances on coyotes, but its just not been the case for me. Maybe the fast twist on my new rifle will give me the results many folks enjoy when using the 223 on coyotes /ubbthreads/images/graemlins/grinning-smiley-003.gif
 
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