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Twist and Spin
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Ominivision1
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PostPosted: Thu Oct 14, 2010 7:06 am    Post subject: Twist and Spin Reply with quote

Was reading a book called firearms and ballistics by brian heard and I knew that bullets spin very fast but never gave it much thought till I read the book. Here's an example:

Muzzle Velocity × 720 divided by twist = rpm

For example:
9 mm bullet at 1200 fps fired in a barrel with a 1 in 10 twist rate will have
a rotational speed of 1200 × 720/10 = 86,400 rpm

0.223′′ bullet at 3000 fps fired in a barrel with a 1 in 12 twist rate will have a
rotational speed of 3000 × 720/12 = 180,000 rpm

Amazing how these things don't fly apart when they exit the muzzle.

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Vince
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PostPosted: Thu Oct 14, 2010 11:52 pm    Post subject: Re: Twist and Spin Reply with quote

Ominivision1 wrote:
Amazing how these things don't fly apart when they exit the muzzle.

Oh, but they do on occasion mate. Check out this Utube video. The first group of bullets don't make it any further than about 10 metres before disappearing in a cloud of lead dust.

Bullets disintegrating

Cheers, Vince

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Elvis
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PostPosted: Fri Oct 15, 2010 1:03 am    Post subject: Re: Twist and Spin Reply with quote

push a standard projectile too fast and it blows up or I reckon more like gets torn apart by centrifugal force.

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Ominivision1
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PostPosted: Fri Oct 15, 2010 2:04 am    Post subject: Re: Twist and Spin Reply with quote

Vince:

Watched the video, it boggles the mind sometimes as to how fast these things spin, and yet I have recovered cast bullets that were shot through a handgun and found traces of lube on them!!!

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Vince
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PostPosted: Fri Oct 15, 2010 2:53 am    Post subject: Re: Twist and Spin Reply with quote

Ominivision1 wrote:
Vince:

Watched the video, it boggles the mind sometimes as to how fast these things spin, and yet I have recovered cast bullets that were shot through a handgun and found traces of lube on them!!!

Yep...when I was into pistol shooting, I regularly found lube on the target face and on projectiles picked up down range. But let's not forget that we don't drive cast boolits anywhere near the velocities that most rifle bullets exit the barrel. To be honest, I never checked the twist of my 586 S&W .357 MAG revolver, but even so, I would be extremely surprised if anything I fired from it exceeded 1600 fps...even my earschplittenloudenboomer loads.

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TheBigJonson
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PostPosted: Fri Oct 15, 2010 6:26 am    Post subject: Re: Twist and Spin Reply with quote

Ominivision1 wrote:
9 mm bullet at 1200 fps fired in a barrel with a 1 in 10 twist rate will have a rotational speed of 1200 × 720/10 = 86,400 rpm

interesting...and it sounds huge but since the time to target is only a little more than one-tenth of a second, the bullet will only rotate 150 times or so before it hits the target.

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Pumpkinslinger
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PostPosted: Fri Oct 15, 2010 8:09 am    Post subject: Re: Twist and Spin Reply with quote

Remember that old BS about Black Talon bullets "cutting Like a buzz saw" as they spun? If you do the math you see that they'd probably make one turn or less in the target. If the bullet makes one turn in 10" and penetrates 10" its not exactly spinning like a buzz saw.

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slimjim
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PostPosted: Fri Oct 15, 2010 9:24 am    Post subject: Re: Twist and Spin Reply with quote

Thus, the benefits of monolithic bullets when achieving high velocities in high-twist barrels.

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Suzanne
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PostPosted: Fri Oct 15, 2010 8:00 pm    Post subject: Re: Twist and Spin Reply with quote

That was educational, I thought someone had seen me on the stripper pole spinnin and twistin......least they don't have the video on here....

Suz

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PaulS
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PostPosted: Fri Oct 15, 2010 8:51 pm    Post subject: Re: Twist and Spin Reply with quote

Now THAT is something!
Not Suz on a pole - though that might be fun to watch but I was talking about disintegrating bullets. A range aquaintance of mine used to do that with his swedish mauser. I think he was using a modified case with more capacity but it was a long time ago.

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Suzanne
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PostPosted: Sat Oct 16, 2010 8:04 am    Post subject: Re: Twist and Spin Reply with quote

Would probably not be fun to watch!

I didn't know bullets could do that, my desktop dental handpiece rotates up to 400,000 rpm, but the typical use is around 30,000 rpm. Safety glasses!!

Suz

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Ominivision1
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PostPosted: Sat Oct 16, 2010 8:53 am    Post subject: Re: Twist and Spin Reply with quote

Was finishing up the article and came across something else, I always thought heavier bullets required more twist to stabilize, but apparently bullet weight has nothing to do with twist requirements!! Bullet length is the major factor, it gave me a website to go to with I did and entered the figures for couple of bullets I reload and it came up with proper twist every time. Check it out for your self.

kwk.us/twist.html

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Ominivision1
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PostPosted: Sat Oct 16, 2010 9:19 am    Post subject: Re: Twist and Spin Reply with quote

To add more fun to this, I was looking at Winchesters model 70 extreme rifle in their catalog and am stumped as to why the .308 takes a takes a 1 in 12 twist and 30/06 takes a 1 in 10. These are the same boollits. Granted there's a 2" barrel length difference, but why the longer twist for the shorter .308 barrel?

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Last edited by Ominivision1 on Sat Oct 16, 2010 10:15 am; edited 1 time in total
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Aloysius
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PostPosted: Sat Oct 16, 2010 10:09 am    Post subject: Re: Twist and Spin Reply with quote

10" is shorter than 12", so 1 turn in... means here the .308 Win has a longer twist, maybe because he's meant for lighter (or shorter) bullets + the 30.06 normally can give more fps.
Look at the .300 WM, most of them also in 1 in 10" and the 30-30 Win with most barrels 1 in 12" and some (Marlin) 1 in 10".

One might conclude that for jacketed bullets till 150 grs a twist of 1 in 12" is OK and for heavier bullets the 1 in 10" is recommended.

Just to check: my Winchesters in 30/30 like the 125 and the 150 grs bullets, who is using the Marlin in 30/30 and does he prefer the 170 grs?
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Ominivision1
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PostPosted: Sat Oct 16, 2010 10:20 am    Post subject: Re: Twist and Spin Reply with quote

Aloysius wrote:
10" is shorter than 12", so 1 turn in... means here the .308 Win has a longer twist, maybe because he's meant for lighter (or shorter) bullets + the 30.06 normally can give more fps.
Look at the .300 WM, most of them also in 1 in 10" and the 30-30 Win with most barrels 1 in 12" and some (Marlin) 1 in 10".

One might conclude that for jacketed bullets till 150 grs a twist of 1 in 12" is OK and for heavier bullets the 1 in 10" is recommended.

Just to check: my Winchesters in 30/30 like the 125 and the 150 grs bullets, who is using the Marlin in 30/30 and does he prefer the 170 grs?

Aloysius:
I corrected my statement, I meant to say longer twist. I have a Marlin 30/30 and it like heavier bullets like 170gr than 150gr as my Marlin has a 1 in 10".

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