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twist rates and end results
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slimjim
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PostPosted: Tue Dec 20, 2016 3:03 am    Post subject: Re: twist rates and end results Reply with quote

First, it is centrifugal force that can cause a lead-core bullet to disintegrate in flight. It’s not barrel twist and it is not velocity, it is both combined – twist rate x velocity = RPMs. A barrel with a slower twist rate can spin a bullet at a higher RPM if shot at a higher velocity.

The first indication I found that bullet spin rate affected bullet expansion was with the Barnes .277 110 TTSX. A hunter was claiming great results in the 6.8mm SPC and showed a beautifully expanded bullet he had recovered while the expansion I was getting was dismal. The only difference was spin rate. I confirmed this with a terminal performance test shooting the 110 TTSX from a 1:11.25 and 1:10 twist at the same velocity. The .270 Win is typically a 1:10 twist and the 6.8 is 1:11.25 plus the .270 launches the bullet at a higher velocity (3400 vs 2700 fps) resulting in a spin rate of 244,800 RPM vs 172,800 RPM, respectively. The added centrifugal force helps the TTSX bullet expand to larger diameters. Based on this finding, I did some dedicated testing with .277 and .223 bullets.

I did not find a difference in expansion vs bullet RPM with the Barnes TSX (non-tipped) bullets. I've tested the TSX and TTSX across the caliber spectrum from .223 to .308 and the TSX has always out-expanded the same weight TTSX in the same caliber (does not apply to Barnes new LRX). This is probably why Remington chose the TSX for their Hog Hammer ammo line.

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slimjim
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PostPosted: Tue Dec 20, 2016 3:04 am    Post subject: Re: twist rates and end results Reply with quote

So, right off the bat, I found bullet spin doesn't affect the expansion of every bullet the same. The .223 has even a wider range of twist rates so I did some focused testing there. . In the .223 where twist rates can vary from 1:7 to 1:11 or slower, I found the RPM effect on bullet expansion can be dramatic and not measurable. In the case of Hornady's new 55gr .223 GMX bullets, it will expand as much as 25% more when the bullet is shot from a 1:8 or faster compared to a 1:11 (see graph below). You can also see that the 55gr GMX bullet became elastic just before it lost its pedals around 1100 ft-lbs of energy resulting in some amazing expansion with the faster twist rates. I also tested some Nosler 64gr Bonded and Federal 62gr Fusion bullets. These are bonded lead core bullets and there expansion was not affected by twist rate – each bullet looked like a clone of the one before.



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Last edited by slimjim on Tue Dec 20, 2016 3:12 am; edited 1 time in total
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slimjim
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PostPosted: Tue Dec 20, 2016 3:05 am    Post subject: Re: twist rates and end results Reply with quote

Also, note barrel twist rate only stabilizes bullets enough for flying through the air, not during the terminal phase penetrating through flesh. For a bullet to remain stable and penetrate on-course after impact, the bullet has to shorten and increase/expand in diameter on contact. If not, the bullet will immediately tumble and 1) break in the middle (fragment), e.g., 110 BTHP, 115 Nosler CC, and other OTM bullets ... or ... 2) yaw 180 degrees and penetrate tail first, e.g., .277 130gr VLDs. Even if the bullet expands and shortens it might not be enough to remain stable during penetration. If you take a recovered bullet, measure its length and diameter, and run it through a stability calculator using velocity at impact. The stability factor is around 40 after impact vs 1.5 for stability in-flight. More centrifugal force (faster twist rate) can increase that diameter of expansion for some bullets which should result in a higher rate of energy transfer and likely reduce penetration.

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PaulS
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PostPosted: Tue Dec 20, 2016 6:23 pm    Post subject: Re: twist rates and end results Reply with quote

Slimjim,
I will easily accept your findings and they seem to correspond to the testing I did with partition, bonded and hot core bullets. I didn't do much testing outside of thirty caliber because I was looking for a good hunting bullet for myself (body shots) and my brother(head and neck shots).
The part I don't fully understand is the bullet disintegrating in mid air. I know (in my mind) that centripetal force has to have a part in it but it also seems to be related to frictional heat while traveling in the air. We know the bullets get hot enough to melt or vaporize the plastic tips but do they get to a temperature that is critical to the bullet? I don't remember how to calculate the forces or stresses on a spinning body. I'l have to see if I can take the time to do it. I do know that the radius from the center of mass to the center of mass of the rotating part and its angular velocity all comes into play but I will have to look it up - been a long time ...

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Elvis
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PostPosted: Tue Dec 20, 2016 7:33 pm    Post subject: Re: twist rates and end results Reply with quote

great stuff as always Slim...thanks mate.

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slimjim
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PostPosted: Wed Dec 21, 2016 2:27 am    Post subject: Re: twist rates and end results Reply with quote

PaulS wrote:
centripetal force has to have a part in it but it also seems to be related to frictional heat while traveling in the air.

The friction in the barrel - metal to metal - has to be more excessive than when traveling through the air.


PaulS wrote:
We know the bullets get hot enough to melt or vaporize the plastic tips

This is only for bullets shot at high velocities that have high BCs that can maintain their velocities for a long distance (beyond of 300 yards). The tips don't vaporize, the melt and deform. When a bullet disintegrates, it occurs just a few feet in front of the muzzle where the centrifugal force is the greatest.

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PaulS
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PostPosted: Wed Dec 21, 2016 1:16 pm    Post subject: Re: twist rates and end results Reply with quote

Slim sometimes the bullet can make it to 100 yards but blows up between there and 200 yards. The rate of spin doesn't slow much even at long ranges.
Hey, you could be right but as you say if it was just the spin then it should happen on exiting the barrel or shortly there after. A larger diameter bullet would be more likely to be destroyed by spin forces because of the larger diameter but you I have never hear about anything but small diameter bullets.
There is something more that I am not considering here and its making me loony.

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slimjim
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PostPosted: Thu Dec 22, 2016 4:38 am    Post subject: Re: twist rates and end results Reply with quote

PaulS wrote:
Slim sometimes the bullet can make it to 100 yards but blows up between there and 200 yards.

I have only heard/seen videos of bullets disintegrating shortly after they leave the muzzle.

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PaulS
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PostPosted: Thu Dec 22, 2016 12:33 pm    Post subject: Re: twist rates and end results Reply with quote

That helps to explain your experience, thanks.

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