1:10 vs 1:12 Twist Rates

[Delmar]

Bought some new reloading manuals and it seems that new manuals are basing their loads in 308 Winchester with a 1:10 twist rate, rather than the 1:12 on the rifles I have.
Generally speaking, I get my best accuracy with loads a bit shy of maximum recomended, but I like to have the data match closely. For example, if the load calls for winchester cases with a particular primer/bullet/powder, pressed to a particular length, thats exactly what I will assemble, at least as a starting point. I suppose the slower twist will cause a slightly lower pressure, but is there a corrolation here?

 

[Dave P]

I suppose the slower twist will cause a slightly lower pressure, but is there a corrolation here?

I don't believe any corrolation: the max pressure (I assume) is experienced just before the bullet leaves the case; maybe 1/4" into the lands, in most examples.

I doubt if you could measure the pressure difference based on 9 degress rotation (1:10) versus 7.5 degrees (1:12); the twist isn't part of that pressure equation.

The 308 is a very easy and forgiving rnd to load for, I see no reason to get concerned with this minutia; use the books as guide, and have fun finding the load that performs the best for you.

 

[uglymofo]

Agreed. My only advice would be that your 1/12 will probably perform better with bullets weighing less than 173gr. The 168's and 155's are supposedly ideally suited for the 1/12.

 

[Steve Smith]

No offense, uglymofo, but a more precise way of saying that would be "The 1:12 twist is more ideally suited for 168s and 155s."

Delmar, such a small twist rate change should not make a significant difference in pressure. I think the difference between a 1:12 and a 1:6.5 in a .223 WOULD make a difference.

 

[uglymofo]

the horror of French class....

No offense, uglymofo, but a more precise way of saying that would be....

I remember getting hammered in French class for years. I couldn't speak it, or write it, but I could read and understand it with the best of them (in my classes). Then one of my teachers is helping me after school and we find out that I dunno squat about semantics, grammar and sentence structure; yet I was always a finalist for the class spelling bees....I stand corrected, both for grammar and semantics.

 

[Delmar]

uglymofo

....I stand corrected, both for grammar and semantics.

Hope my question didn't make you stay after class again
Glad to know this twist rate business is a minor annoyance, and a couple of my new manuals do not even list a twist rate, so back to the bench I go.

I had also heard the 168 grain was very compatible with the 1:12 twist rate, although it also does very well with flat based 180 grain Hornadys.

My oldest son just graduated from high school and reports for basic training at Ft. Benning, GA this fall. Asked him what he wanted for a graduation present and he said "your M700 VS", which I have had for better than a decade. He didn't want a new one-told me the load development was already done on it. Smart kid-maybe too smart.....

So, I am starting all over again on a brand new 700VS-one thing I noticed, other than the key start on the bolt, is the crown. It's identical to the 700P-slightly recessed and flat compared to my old VS which is concave. I did get back at my son in a way. My old one has a Simmons 6.5-20 White tail classic, and the new one has a Leupold 6.5 x 20 Vari-X III.