Let's do the twist.... :)

[JAS-SH]

Hornady just dropped a new podcast on twist rates and the consequences of too much or too little. Complex but if you want to know everything you need to know about twist rates, this is it.

Thing's like:

- 300,000 RPMs is the do not exceed limit. Lower is better, within reason. RPM formula is in the podcast. My 6mm ARC bolt rifle with a 1:7 twist at 2750 fps with 108gr ELD Mach bullets is doing 282,857 RPMs. Darn close to the max. What that means is that if I want to go out and shoot varmints with lighter bullets I might be in trouble.
- Most formulas do not include air density. The colder it gets where you shoot, the lower the twist you should use.
- Most of the the twist rates bullet / cartridge manufacturers twist rate calculators use older formulas with a 20~30 percent error.
- Gyroscopic stability should be at least 1.4 at the muzzle (4DOF calculates that) . Bullets get more stable the further (slower) they go.
- The slower the twist rate (within minimum twist rate) the smaller the dispersion, which happens early on the bullet's flight.

Tons to see here:

 

[Mr. F]

#airburst

IYKYK

 

[srt-4_uk]

Do they have a separate formula for 147 ELD-M?

 

[Sig Marine]

Simple formula: muzzle velocity times 720 divided by twist rate

 

[Huskydriver]

Do they have a separate formula for 147 ELD-M?

And the 140 and the 225.....

 

[JAS-SH]

Do they have a separate formula for 147 ELD-M?

If you are referring to RPMs the formula is the same for all caliber/twist combinations. There is another formula that has to do with different calibers, which is important as well, and that is towards the end of the podcast.

The RPM formula is 720 x Muzzle Velocity / Twist rate. You do need a chrono though to get the exact velocity numbers.

With my 6mm ARC rifle with 108g ELD Match those numbers are: 720 x 2750 / 7 = 282,857.14 RPMs.

Still within the limits, but not for say an 87 grain VMAX Varmint bullet. That one comes out at 334,285.71 RPMs I used the bolt action max load from Hornady's reloading manual at 3250 FPS. At those RPMs the bullet could possibly disintegrate in mid air.

For comparison, if the twist rate was 8 the RPMs would be 292,500 RPMs. Right at the limit.

Hornady though recommends a minimum of 8 twist for the 108 ELD Match.

And, longer barrel lengths also might be detrimental in the equation because they also increase bullet temperature - can make bullets fail quicker.

And, air density also has an effect (as in cold temperatures)!

All in all a great podcast!

 

[mullmann]

Whoops, Guess I should have gone with a 7.5 twist on my 25PRC and the 6.5 barrel I just ordered for a 6.5-7prc. 313k and 308k.

 

[Nik H]

Robert Gradous used to chamber all of my barrels and he recommended 7.5 twist for 6.5 CM. I have never looked back

 

[Potsy]

If I did my math right I’m 313k for 88’s in my 1:7 .22BR. No problems so far with them or 75’s at a bit more.
I reserve the right to change my tune in another thousands rounds.

 

[JAS-SH]

Concentrating on RPMs is not the point. RPMs should not be too high but they will really matter if you have a fast twist barrel using heavy for caliber bullets and then switch to light bullets as in varmint style ones which are more frangible.

The more important thing is Gyroscopic stability. Hornady has a technical paper on the 4DOF calculator that explains a great number of things. One of them is Gyro. That explanation is below.

The calculator has other important things like automatic velocity adjustments based on powder temperature sensitivity and more. Pretty cool stuff.

I've attached the PDF for the technical paper below. Here's the Gyro explanation:

Page 6 on the 4DOF technical paper:

Gyroscopic Stability​

Gyroscopic stability (Sg) is merely a measure of the projectile’s ability to maintain point forward
flight and not assume a large and variable angle of attack. A minimum gyroscopic stability factor at the
muzzle of 1.0 is required for a bullet to fly point first. Sg depends on the projectile’s mass, moments of
inertia, spin rate, air density, pitching moment and velocity. Existing stability calculators available today
based off bullet length are a good rule of thumb estimate, but they are exactly that, approximations.
Without properly modeling mass distribution inside the projectile as well as the effect its unique shape
has on the location of the Normal Force Center of Pressure location, accurate gyroscopic stability
calculations using the Greenhill or Miller stability calculations are estimates. The Hornady 4 DOF
accurately calculates Sg based on each projectiles mass, aerodynamic properties, and atmospheric
conditions. It must be pointed out that a projectile gets rapidly more stable gyroscopically as it flies
downrange. The spin of a projectile decays at a much slower rate than its axial velocity does. The changing
aerodynamic properties as the projectile slows, without the spin appreciably changing, results in a more
and more stable projectile as it flies downrange.

In general, a projectile with Sg values at the muzzle, in ambient atmospheric conditions, of around
1.4 is considered the lower limit. This allows for some error in the calculation of the Pitching Moment and
for increased air density when the projectile is fired under cold conditions at low altitudes. Hornady 4 DOF
will display the Sg of the bullet as it flies downrange. If you have an Sg of less than 1.4 at the muzzle under
ambient conditions we would recommend you model a faster twist rate, and consider using a faster twist
barrel. Extensive Doppler radar testing has shown that for supersonic Mach numbers above 1.7 - 1.8 that
the drag of a projectile reaches a minimum at an Sg of about 2.0. The transonic drag on a projectile will
continue to decrease as the spin rate and Sg is increased. There are practical limits of this as you can spin
a projectile to the point that it will mechanically fail in flight from excessive centrifugal force. Excessively
spinning a non-expanding bullet will have detrimental effects on its terminal performance.

After running a trajectory, the first check should be the highlighted “Gyro” column of the outputs
table, see Figure 2. The Hornady 4 DOF will not produce an error if the Sg is below 1.0 at the muzzle.
Instead, the 4 DOF will model the projectile as unstable until it has lost enough velocity to climb back to a
Sg of 1.0. When launched with a Sg of less than 1.0, velocity loss occurs extremely rapidly and should
appear abnormal. Figure 3. is an example output table showing the highlighted area that should be
checked by the user to ensure a properly stabilized bullet. This value should be checked for each trajectory
ran.​

 

[jetjock07]

If I did my math right I’m 313k for 88’s in my 1:7 .22BR. No problems so far with them or 75’s at a bit more.
I reserve the right to change my tune in another thousands rounds.

Yeah I’m at 307k with mine running 88’s

 

[Acronin243]

If I would have read this post before posting my question I may have refrained. But I guess my question still matters in terms of how people deal with marginally stable bullets.

But I'm guessing with this calculation if I reach RPM's in the 260,000 I'm sitting pretty good with what I've got?

.22-250AI shooting 75gr ELDM's at 3250fps....