Twist and Spin

Ominivision1 · Posted: Thu Oct 14, 2010 10:19 am Post subject: Twist and Spin

Read an interesting article in a book I have regarding bullet rpm after exiting the barrel.

Muzzle Velocity × 720 divided by twist rate of barrel = 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 these don't fly apart after exiting the muzzle!!

slimjim · Posted: Wed Nov 03, 2010 8:43 pm Post subject: Re: Twist and Spin

Ominivision1 wrote:

Amazing these don't fly apart after exiting the muzzle!!

They do! Try some light .223 40gr in a 1:7 twist.

SingleShotLover · Posted: Thu Nov 04, 2010 5:02 am Post subject: Re: Twist and Spin

Hornady 50-grain SX bullets from a 1-9" twist at 3400fps have a pretty short life expectancy too!

slimjim · Posted: Thu Nov 04, 2010 2:27 pm Post subject: Re: Twist and Spin

SingleShotLover wrote:

Hornady 50-grain SX bullets from a 1-9" twist at 3400fps have a pretty short life expectancy too!

You must have a long barrel because I don't get anywhere near that velocity out of my 16" 1:9 which doesn't have a problem with the lighter bullets.

chambered221 · Posted: Thu Nov 04, 2010 5:12 pm Post subject: Re: Twist and Spin

Ominivision1 wrote:

For example: 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

This is true but keep in mind that the bullet will never see that much rotation !!!

Divide that by 60 (180,000/60=3,000) to get a time of flight of 1 second.
A 55gr bullet traveling 3,000fps takes just shy of 1 second to go 600 yards, should it make it 1,200 yards it still only made 6,000 revolutions.

None the less bullets do disintegrate when enough velocity and twist are applied.

Elvis · Posted: Thu Nov 04, 2010 11:24 pm Post subject: Re: Twist and Spin

yip the g forces and centrifugal force are great

SingleShotLover · Posted: Fri Nov 05, 2010 6:46 am Post subject: Re: Twist and Spin

slimjim wrote:

You must have a long barrel because I don't get anywhere near that velocity out of my 16" 1:9 which doesn't have a problem with the lighter bullets.

Using a less than maximum charge of Accurate Data Powder 2200 (long discontinued, but I have a generous supply) I reach just over 3500 fps (chronographed) in my 26" .223 Ruger Number One with a 1-12" twist, but now only use the VMax bullets which are tougher. In my friend's Savage with a 22" barrel and a 1-9" twist, the same loads reach 3387 fps but seldom make it to the 100-yard line. Most shots just result in a gray trail in the air. This load shows no adverse pressure signs in either rifle. I figure that even though the SX bullets at that velocity work in my rifle's 1-12" twist rate (and are quite accurate) I am probably treading on the verge of bullet destruction which is the main reason for my switch to the VMax bullets.

Actually, I learned this lesson quite a few years back when I pushed the SX bullets past 3,600 fps in a 1-14" twist .22/250 and suffered bullet destruction. It's not that the SX bullets are bad bullets...it's just that they are intentionally made with thin jackets to promote violent expansion at moderate velocities. Speeding them up past a certain point or twist rate is asking more than is reasonable.

Just my experiences...I'm sure others will have different ones.

slimjim · Posted: Fri Nov 05, 2010 7:47 am Post subject: Re: Twist and Spin

SingleShotLover wrote:

Just my experiences...I'm sure others will have different ones.

I wasn't disagreeing at all with you. As I expected, you are getting those velocities with longer barrels. I'll have to watch out for that now that I have just acquired a 24" with 1:8 twist. Though all my 40gr V-Max's were making it to 75 yards with a muzzle velocity of 3700 fps.

chambered221 · Posted: Fri Nov 05, 2010 5:45 pm Post subject: Re: Twist and Spin

Why are you running 40gr bullets in a 8 twist ???

Alll the posting you've done on twist and stability factor I would have thought you had this all figured out by now !!! Viking

SingleShotLover · Posted: Sat Nov 06, 2010 6:44 am Post subject: Re: Twist and Spin

No problem slimjim...I didn't take it that way. Each rifle is an individual and it's hard to know what works within certain parameters unless you try!

slimjim · Posted: Sat Nov 06, 2010 3:40 pm Post subject: Re: Twist and Spin

It is an impressive sight to see a bullet disentigrate in a "poof" on the way to the target.

PaulS · Posted: Sat Nov 06, 2010 4:53 pm Post subject: Re: Twist and Spin

slimjim wrote:

It is an impressive sight to see a bullet disentigrate in a "poof" on the way to the target.

Its a lot more impressive to see a prairie dog disintegrate in a spray of blood as the bullet hits the target..... Just sayin'

Elvis · Posted: Sun Nov 07, 2010 12:20 am Post subject: Re: Twist and Spin

hornady 50grn sx are great wallaby and hare medicine at 3000-3200fps in my zastava they were accurate and great hunting loads. my loads are comfortable to shoot and easy on the ears. Ive not seen the need to speed them up any more.

44marty · Posted: Sun Nov 07, 2010 9:51 am Post subject: Re: Twist and Spin

You can disintegrate bullets with insufficient twist as well. A bullet too long and heavy fired from a barrel with a twist rate that isn't tight enough to stabilize it can tumble and disintegrate.