Twist rates ... a calculator to better find the right gun??

jb4lcm

I was poking around trying to better understand what ammo and twist rates go together when considering a new gun. I came upon this calculator that uses some SIMPLE math and much to my surprise - I am seeing possible answers as to why my previous gun - Ruger American 30-06 with 22" bbl at 1:12 twist may have been having problems ... (I sold the ruger so ...)

This calculator uses the following equation: (CxD2)/L=twist rate.

"C" is fixed. For ammo UP TO 2800 FPS C=150. For faster ammo C=180.

"D" = the diameter of the projectile in inches - squared. Do the math on MM rounds to get to inches (xmm/25.4)

So a 30 caliber round is .308" x .308 = .0948" is the "D" number here.

"L" = the length of the bullet in inches. I have found a couple of pages that have some info on this:

Winchester Power Point 150gr = 1.047" long @2920 FPS; 180gr = 1.365" long @ 2700 FPS

So if I do the math I find this:

150gr Winchester power point @ 2920 FPS = (180x.0948)/1.047" = 1:16.3 twist ?? really? I cannot find a 30-06 rifle with a 1:16 twist. Lighter/shorter (faster) loads will need even slower twists.

180gr Winchester power point @ 2700 FPS = (150x.0948)1.365" = 1:10.4 twist - This I can find!

Heavier rounds will be longer so the twist rate will increase to a faster twist, I just don't have the data on heavier loads at the moment ... anyone know the length of a 200gr or 220gr round?

Most of the "typical" rifles in 30-06 I have found are 1:12 (like my Ruger American). Some are 1:11 and others 1:10 (Browning BAR).

An interesting fact here: Muzzle velocity of 2700 FPS = 162,000 RPM ...

No wonder the comment "you gotta find the round your gun likes" is starting to makes sense now.

What's your take?

truthful

Seems it would be useful to use the equation the other way around. Enter the twist rate of the gun you own or are considering, and compute the bullet length, ie, bullet weight.

select-fire

Experiment. My 300 Win Mag is zeroed at 250yd with 180 gr. Shoots 2.5" high at 100 yd. Now shooting the same ammo in 150 gr. it is 6" high at 100yd. Making it dead on at 450 yd.

kidthatsirish

I would say for the 30-06 now slower than 1:12 and no faster than 1:10. When in doubt go with the faster rate...it's a lot harder to over stabilize than under stabilize.

jb4lcm

Okie - it's the total - overall length. We are talking about stabilizing the round outside the bbl - length/weight has different mass and response depending on length. 350gr ROUND BALL with .5" diameter = .5" length and will respond differently than a 350gr boat tail shape. So overall length is what is measured even if the tapered * end doesn't touch the bbl, its length changes the wobble/yawl effect. So that being said - solid copper is lighter than lead which will be longer (same caliber) at 150gr than FMJ or jacketed SP.

if that makes sense.

Ambrose

You asked for bullet lengths so I checked a few:

200 gr. Sierra boat tail Gameking= 1.384"

200 gr. Sierra boat tail Matchking HP= 1.394"

220 gr. Sierra boat tail Matchking HP= 1.485"

220 gr. Sierra round nose Pro-Hunter= 1.309"

220 gr. Hornady round nose= 1.269"

The standard twist rate for the .30/06 is 1 in 10". It's a carry-over from the 220 gr. round nose bullet originally used for the .30/03. Interesting that the .308 which is a bit slower in velocity has a standard twist of 1 in 12" as does the .30-30 but they were designed to use shorter bullets.

In reading old American Rifleman magazines from the 1930's, the theory then was that the best accuracy was obtained with a bullet that JUST stabalized. Barrels must have been a lot less expensive then since the experiments sometimes involved changing barrels to get as little as 1/2" difference in the twist.

62vld2042

Over the years, I've found the JBM Ballistics website to be very helpful.

grdad45

JMO---When a bullet is spinning at 150-200K RPM, I would think a solid copper one would have less chance of being out of balance. That should take out one variable concerning accuracy at long ranges.

Mr. Perfect

Where did you find that calculator? I am suspect of the C values.