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280 Ackly Build Twist rate




Deerbegone

1shot counts
#1
OK...working on a build that I've wanted to do for awhile. I'm going with a quality chassis, a trued up Remington 700 Stainless long action, Criterion Varmint barrel...24-26", Shilen trigger. Chambering will be .280 Ackly Improved. I'm likely going to be hurling 165-175ish pills and I'm looking for opinions and reasons for the barrel twist rate. I want to be versatile for different grain weights in case I want to go a little lighter or a little heavier on the actual bullets. I'm thinking 1-8.5 should give me some options. Any thoughts??
 

Deerbegone

1shot counts
#3
I know it depends on the bullet but 8 or 7.5 might overdue the lighter variety in 7mm. Although I plan on developing a couple loads/nodes, a load for 150ish or less grain 7 mm's might over stabilize the lighter variety. My target bullets (180ish+) will be on the heavier side for this cartridge. I'm gonna try to find the best of both worlds.
 
#4
My 7.5 does great with 162 & I think 185. Can't remember what bullet I used. Not at bench right now. Under 162 you may want to stay with 8.

BW
 

NYECOGunsmith

Obsessed Member
Staff member
Moderator
#6
You can't over stabilize.
You can however spin the light and fragile varmint bullets fast enough to take them apart in the air.
True true!
When you spin the bullet too fast, the jacket tends to heat up, expand, and then get pulled outward by the centripetal force, tearing the bullet apart.

To some extent, you can speed a bullet up and use a slower twist rate and still get it up to enough RPM's to stabilize it, so you can have some versatility in bullet weight with nearly any twist rate just by varying the velocity of the projectile.

The formula for calculating the RPM of a bullet is simple, MV X (12/twist rate in inches) x60 =RPM, where MV is the Muzzle Velocity of the projectile.

Most bullet manufacturers recommend a twist rate for each of their offerings in the way of projectiles, but you may have to dig a bit for that info from them.

You can also calculate the twist rate required to stabilize using the old Greenhill formula, and get pretty close, remember to use 150 as the constant for projectiles moving less than 2,800 FPS, and 180 for ones moving more than 2,800 FPS.

The original Greenhill formula was:
Twist = C * D * D / L where

D = Diameter of the bullet in inches
L = Length of the bullet in inches
C = A constant (defined to be 150 in the original equation, but as I said, use 180 for more modern, higher speed projectiles).

There is a more modern version of Greenhills Formula that takes into account the material the projectile is made from, where Greenhills just assumes lead, or lead with a gilding metal jacket.
That formula is:
Twist = C * D * D / L * Square Root of (SG / 10.9) where
SG is the Specific gravity of the bullet material.
For a lead core bullet, SG = 10.9, which would mean the right part of the equation is equal to 1.

For other materials, the value of SG varies, for example, for Copper it is 8.5, Brass is 8.9, Steel is 7.8.

So with the above you can look up the projectiles you want to use, and do some rough calculations using the velocities you want to launch each of them at, and get an idea of what twist rate will come closest to a "do it all" rate for your application.
 
#7
Sorry....was out of "service"for a minute. No you can't "over stabilize most bullets per say but you can greatly increase the temperature created by surface friction which will adversely affect many bullets...some more than others. I'm not a bullet engineer but some say it can affect yaw rate as well when bullet temperatures and expansion begin to occur especially as the bullet starts to lose velocity reducing friction/ temperature in flight... IDK While I don't plan on shooting 7mm projectiles fast enough to "rip the skins" off of them.... In a hunting application it's pretty insignificant but long range precision could just create another small detail to overcome. Good information from NYECOGunsmith.
 
#8
"...but long range precision could just create another small detail to overcome..."

In long range competition the fast twist rate would be necessary for the long, high-BC bullets.
That's especially where the fast twist barrels are used.
 
#9
Absolutely FlyZeb. Thus my original post. The closest thing I can come to the best of all worlds is 1 : 8.5 twist rate, 26" barrel.....bullets between 150-175 grain ELD-X Hornadys or a similar combo of Bergers' VLD'S.....Sierra's etc. Everything's a trade off for something else. I appreciate the input and ALL the insight I can get.