1 in 12" is a slower twist than 1 in 9", which is a slower twist than 1 in 7". The lower the X in "1 in X twist", the faster the rate.

 

An SL8 has 6x land and groove rifling, 1 in 7" twist. And while everyone uses bullet weight when talking about optimum twist rate, it is actually the length of the bullet that determines the best rate. Weights are used since you know them when you buy ammo, and the weight is a factor of the bullet length.

For educational purposes, here's a link with the formulas and technical explanation, it's interesting:

http://www.uslink.com/~tom1/twistrate.htm

 

I thought the SL8 has a polygonal barrel?

 

FWIW, I thought the faster twist bbl liked heavier bullets, but to my surprise, my 1/7 twist G36K bbl (with pinned 4" Urbach flash hider) likes the 55gr the best for groups.

So with every rifle, I bring several types of ammo to try out and then label the gun with what it's fav is.

You never really know until you try it out. 55gr. 62gr. 69gr 75gr.

It's like a box of chocolate