Bullet stabilization, strictly RPM?

[Michael Eichele]

Message deleted.

 

[BigDaddy0381]

I read above.the when a bullet slows down the the spin stays the same.
So if i get this right a 1.10 twist the bullet spnis 1 time in 10 inches correct?

Okay if it is traviling at 1000fps the bullet is still spining 1 in ten inches just going in a faster rpm at the muzzle.If when it gets out to say 200 yards the fps is now 800 the bullet should still be spining at 1.and ten just the fps has slowed and the rpms is slowed also.

So a 1 and 10 twist will stay 1.10 untill it hits something as everything slowes at once.

I think that came out right after reading some of the stuff abouve my brain has went to apple souce

 

[devildoc]

you're missing a vital jump, there. If you have a 1:10 twist that will impart a certain rpm to the bullet. that spin stays relatively constant (it does slow a bit) however as velocity decreases distance that the bullet travels per spin decreases. so though your bullet was spinning once every ten inches at the muzzle it will be spinning for instance once every 8 inches at distance.

 

[BigDaddy0381]

So the spin will slow more or less than the distance travled.

1.10 twist at muzzle of 1000fps spins 1000times untill the fps drops then the rotation stay the same just over a greater distaince?

i think i might have it now.

 

[Mikecr]

No, effective twist rate changes down range.
For example, a 95vld launched at 3000fps and 1:10tw will have dropped to ~2130fps at 500yds(BC of .505).
The bullet RPMs have only dropped off slightly at ~18rpm per fps of velocity loss. So the effective twist rate at 500yds is now 1:7.7

This bullet had an Sg at the muzzle of 1.3, but at 500yds it's Sg has climbed to 1.9

At 1000yds the effective twist would be 1:5.5, and Sg would be 3.8

 

[devildoc]

Okay, I didn't really have time to explain well before so I will now. So if you have a bullet with a muzzle velocity of say 3000fps and a 1:12 twist. that means that the bullet will spin 3000 revolutions per second or 180,000rpm (180krpm). That spin will only slow slightly during flight, not enough to worry about for our purposes. So if at some distance the projectile is only going 2000fps and the bullet still only takes 1/3000 of a second to turn, the projectile will only be going 2/3rds of a foot or 8" per turn compared to the muzzle value of 12" per turn. This corresponds to less displacement per turn. Got it now?

On an interesting side note, alot of people are always talking about how a spinning bullet will do some kind of terminal blender action when it hits, looking at the numbers above kinda illustrates how that just isn't the case. You hit a large hunk of big game animal and the bullets only going to spin a few times at most spread out over a couple feet of tissue, probably much less considering that once expansion initiates that spin will slow rapidly.

 

[Michael Eichele]

On an interesting side note, alot of people are always talking about how a spinning bullet will do some kind of terminal blender action when it hits, looking at the numbers above kinda illustrates how that just isn't the case. You hit a large hunk of big game animal and the bullets only going to spin a few times at most spread out over a couple feet of tissue, probably much less considering that once expansion initiates that spin will slow rapidly.

Except for it really does create more temporary cavitation during penetration. This causes more "shock" and tissue damage. It really is the case. Higher than normal RPM's DO create more shock than more ballanced RPM's. Is it worth over spinning your bullets? IMHO, no. I would rather spin them to a 1.5 stability factor and get the best BC I can rather than sacrifice my BC for a slightly better terminal effect. If you doubt this "theory" you can test this against ballistic gelatin. You will see for your self the cavitation difference. In extreeme cases, the bullets will fragment in dramatic fashion uponn impact instead of hold together due to the ultra high RPM's and the centrifical force of the jacket and the effects that has on the jacket during impact.

 

[devildoc]

Eh, I've heard alot of conjecture about it and not much science. I'd like to see real scientific results actually showing the phenomenon. Sure there's going to be a little more energy in the system if everything else is comparable but you spin one faster. But I can't see it effecting terminal performance enough to cause any appreciable difference, especially when compared to the emmense forces due to momentum. Might intitiate expansion a little bit faster but other than that the forces involved with spinning a small diameter object would be very minimal in comparison to propelling it at supersonic velocities.

(note, I am not a physicist, but I completed college physics with good grades, and stayed at a holiday inn last night)

 

[Jon A]

Might intitiate expansion a little bit faster but....

But what? Making the jacket at the nose extra thin, adding serrations, adding a large hollow point, adding a plastic wedge-shaped tip, etc....doesn't add any energy or momentum to the bullet either.

But real life experience tells us those techniques can dramatically alter the terminal performance of the bullet.

 

[Michael Eichele]

Eh, I've heard alot of conjecture about it and not much science. I'd like to see real scientific results actually showing the phenomenon. Sure there's going to be a little more energy in the system if everything else is comparable but you spin one faster. But I can't see it effecting terminal performance enough to cause any appreciable difference, especially when compared to the emmense forces due to momentum. Might intitiate expansion a little bit faster but other than that the forces involved with spinning a small diameter object would be very minimal in comparison to propelling it at supersonic velocities.

(note, I am not a physicist, but I completed college physics with good grades, and stayed at a holiday inn last night)

If you would like to see scientifec results, test a rifle against another of the same caliber and use the same bullet at the same impact velocity but shoot one with a standard twist, lets say a 175 SMK 30 cal with a 12 twist and then a 9 twist into ballistic gelatin. You will see a big difference in the amount of damage.

Again, is it worth it? It isnt to me, but it is up to the indavidual hunter/ballistician to decide. Whether or not it is worth it doesnt change the fact that it happens. It is just an interesting cause and effect, not neccesarily a better way of killing game.