Twist Rate Question......

[mgd45]

I was going to experiment with doing some handloads for my Remington 700 VLS in .223. The barrel is a 26 inch bull & it is 1/12 twist. I was planning on experimenting with the Hornady 68 grain BTHP bullets but I've discovered that they are only recommended for a twist rate of 1/10 or faster.

Question......how come a barrel in 1/12 twist is good enough to spin a 150-168 grain bullet in a .308, but isn't recommended for a 68 gain bullet? If the twist rates are the same, what is the "technical cause" that makes it different for a .223 bullet?

I mean if a 1/12 twist can spin & stabilize a 55 grain bullet..."but not a 68 grain bullet cause it's too heavy", then how can it spin a 150-168 grain bullet...?

Am I missing something here....? Take it easy on me.....

 

[FNARSHOOTER]

Re: Twist Rate Question......

Bullet lenght v.s. Bullet weight. throw in bullet speed and bearing surface. You are trying to stabilize a projectile in flight. Look up B.C. S.D. and twist rates .
Scot

 

[mgd45]

Re: Twist Rate Question......

Gotcha.....the "Greenhill Formula"......now it makes sense.

 

[FNARSHOOTER]

Re: Twist Rate Question......

cool beans

 

[MitchAlsup]

Re: Twist Rate Question......

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: mgd45</div><div class="ubbcode-body">Question......how come a barrel in 1/12 twist is good enough to spin a 150-168 grain bullet in a .308, but isn't recommended for a 68 gain bullet? </div></div>

A 308 bullet has considerable mass out near 0.300 inches, while a 223 has absolutely no mass out near 0.300 inches. Thus, the 308 round needs less twist than a 223 of ths same weight. This is standard moment of inertia stuff.

In any event, a 1:12 twist is a slow twist rate for 223/5.56 generally good for 55gr and maybe up to 62 gr (lead core) bullets. For 69 gr SMK you want 1:10, for 75 gr you want 1:8 and for 80 gr you want 1:7 and for 90 gr you want 1:6.6.

Given that you have a 1:12, you will want to stick in the low weight ranges until you wear this barrel out and then pop for a 1:9 or 1:8 depending. Unless you are a top ranked shooter the difference in accuracy between a 1:8 and a 1:7 twist rate barrel is (for practical purposes) immesurable up close or way out there. For top ranked shooters, there is just enough gain that being right on the edge of stability can be taken to the bottom line. If you are not one of these chosen few, you loose almost nothing by selecing the faters twist barrel.

In any event to shoot the 75s, 77, and 80s you should be looking at a 1:7 twist barrel (for 80s especially) and for 90s you really want a 1:6.6. MOST of the time you can get away with a 1:8, but dont be surprised if you show up at sea level in 40dF temperatures and find stability problems with the heavies way out there if you choose to push your luck. In hot climates, or at altitude, feel free to push the twist rate considerably.