ok here is a little thing for you guys to do,


Calculate the RPM of a 5.56 mm bullet going at 3000 FPS

in a * barrel

1/7
1/8
1/9
1/2
1/4

Certainly you mean to exclude any drag, nose wind, or other factors that *should* be a part of the equation, right?

If I cant get it from Google, Im not going to play.

Quoted: Certainly you mean to exclude any drag, nose wind, or other factors that *should* be a part of the equation, right?

Since he used muzzle velocity I would assume the calculations are at the muzzle .

Do your own homework

RPM of a fired bullet is often used as media BS.

FLYING BUZZSAWS SPINNING AT CLOSE TO A QUARTER OF A MILLION RPMS!!!!!
(see articles in the mass media about Black Talons for examples)

Nevermind that even if a 55gr 223 rifle bullet were to violate all principals of physics and not yaw it would ,given a frontal upper torso impact, make far less than than two complete revolutions before exiting(1:9 twist) a person of average size.

Thats true Phil_in_Seattle but we are playing with special loads and the bullets explode from the rpm and twist rate being incorrect, we are not concerned with what the media thinks only that our bullet do not explode before they impact the target.

Stop pushing thin skinned varmit bullets so hard and you're less likely to have that disintegration problem.  Reasonable quality solids don't have that problem.    

Excessive spin drift (to the point beyond which aerodynamics wil not keep it in check as is normal until extended ranges are reached and corrections to firing solutions may be used) caused by poor bullets or a faulty muzzle crown can add lateral stress to the bullet that can contribute to it's inflight failure.


Use the Greenhill forumula for your twist starting points and work out from there.

to calculate the twist required to stabilize a given bullet

T=150 x D                                                        
      ---                                                      
       R  

T=twist required

D is the bullet diameter (in inches)

R is the bullet length to diameter ratio, (length divided by diameter)

----

To calculate what length bullet will be stabilized in a given twist, use:

                                                               
L=150 x D x D                                                  
       -----                                                  
         T          
L= bullet length

The constant of 150 is used for velocities 1500-2800fps, above 2800fps use 180 as the constant

i have had JHP and JSP bullets  explode before in reloads

OK, a fictional bullet, traveling at a constant 3000 FPS, with a constant rotation, will travel 180,000 feet in a minute.
In a 1:7" barrel, the bullet will turn 1.714 times per foot, times 180,000

1:7" = 308,520 RPM at the muzzle

1:8" = 270,000 PRM at the muzzle

1:9" = 240,000 RPM at the muzzle

1:12" = 180,000 RPM at the muzzle

1:14" = 154,285 RPM at the muzzle

The exception is, if the bullet is traveling through Phil_In_Seattle, it will stop and wonder what he is talking about.

87 mph.

You can also overspin projectiles and cause overstability. This results in the not-so-desirable condition that keeps the nose of the round pointed high, as illustrated below:

You can also spin them so hard they fly apart.  That's rare, but it happens if you are dealing with very tight twists and very high velocities.  When fired at 3200 fps in a 1-in-7 twist rifle, a round is rotating at over 300,000 rpm when it leaves the muzzle.  Light, thin-jacketed varmint bullets (i.e., 40gr Hornady TNT or Federal Blitz bullets) often can't take that much spin and will pull themselves apart.