Re: 20" Barrel and NOT .308?
I would have to look but I did 2 .260's (one was 20" the other 24") with the same reamer, load, brass, bullet ect. The 24" ran 2780 with the 140 A-max and the 20" ran 2670 to 2690. About 100fps loss.
I would probably go .260 as well. I'm on my 4th that will...