20 Practical Twist Question

[redrockranger]

Heavier bullets faster twist, 1-12 twist 32-39 grain (some 40's depending on gun. 1-11 good to go up to 50 grains. For anything above that, why not just go with the tried and true 1-9 common twist for .223.

Here's what i'm speaking of.

 

[Tommie]

Here's what i'm speaking of.

Precisely

 

[TheCZKid]

1-10 twist runs well with 32 to 40 grain, and if you really want 50 that too.

 

[Coyotefurharvester]

Also, 17,20 cal bullets tend to have lower G1 numbers(they slow down fast) so muzzle velocity is not the only determining factor. You may want to check to see if the twist rate is still stabilizing the bullet at the velocity remaining at max target distance.

 

[moorepower]

Most of the 1-12 twist barrels that don't shoot the 40 grain bullets do not actually measure 1-12. I have a 1-11 on my .20 P and a 1-10 on my .204, but for a 35 grain bullet it will work great. Most 1-12 shoot the Sierra 39 grain bullets also. The 40's need a true 1-12 or faster.

 

[Tommie]

Also, 17,20 cal bullets tend to have lower G1 numbers

Lower than what?

You may want to check to see if the twist rate is still stabilizing the bullet at the velocity remaining at max target distance

Bullet rotation remains virtually constant throughout the flight path. Why would downrange velocity matter in terms of stability?

 

[Bill K]

You would be surprised Tommy. 1/11 is just fine in 20 caliber barrels.

 

[Tommie]

You would be surprised Tommy. 1/11 is just fine in 20 caliber barrels.

Surprised at what?

 

[Bill K]

That the 1/11 will do a fine job of stablizing a 50 grain bullet.

 

[RegionRat]

A Berger 50gr has a length of 0.845", BC of 0.281

If you punch in a velocity of 3700 at sea level with an 11 twist, you get a SG of 0.887 which is showing as unstable.

If you punch in the same as above at 5000 feet, you still only get a SG of 1.07 which is shown as barely marginal. (It is recommending a 1:9.25 twist for the 50 gr, and the 1:10 is giving a SG of 1.29)

I have seen examples of slow twist 22 cals that held stability with longer bullets than the models estimated for a given twist, but I have seen more that actually do fall apart.

The models are easy to run and free on the Berger web site. You just have to dig for a minute to find your particular bullet data if it isn't one of the Berger bullets on their drop down menu.

@Bill K , are you sure you mean the 1:11 twist? What altitude and speed were you shooting at? I'm not saying it (1:11 twist with Berger 50gr) can't work, but I am saying it would be risky.

 

[Tommie]

That the 1/11 will do a fine job of stablizing a 50 grain bullet.

That is correct. I would be surprised.