I was looking at buying a 7x57r in the same fly weight double. JJ at Champlin kind of talked me out of it. My understanding is that it's not that rigidly made. It 's not that it is poorly made, just not very rigid and that as such it tends to be a little less accurate then the second size, especially with a scope.

Size 1 - 7x57r, 6.5x57r, 9.3x74r, and 30-30
Size 2 - 7x65r, 30 blaser and 9.3x74r
Size 3 - 450-400 and 300 win mag
Size 4 - 470 and 500

I'm sure that is incomplete but it's how it was explained to me and might help explain my less then perfect terminology of the frame sizes.

I was intrigued by the fly weight double but I ended going for the one a little bit bigger.