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.
. Not sure why people only use that analogy for energy…
