I'm only somewhat qualified to comment, but thats due to competition. 300-600 is considered mid range, and my Long Range High Master classification came early with ONLY 600 yard and 800-1000 yard matches. 300 and under even 500 and under don't count.
I'll stick by the fact that 69s suck at 600. Its a reason the wife bought an M1A after shooting a windy 600 yard match once, that really took her score down low, and she was handed an M14 the next morning with boxes of 168s( THANKS- Alonda and Arch!!!) and shortly thereafter we had another M1A in the vault.
I would say you're MORE than qualfied to comment. What twist do you prefer when shooting the 80-90 gr bullets. My 24" LB is 1in9 and they highly recommend nothing over 69gr.
my buddy shoots 9 twist on purpose, but its a full and actual 9 twist, hot loads with 80s, sometimes 77s won't stabilize.
I shoot 7 twists for 80s, nothing less than 6.5s for 90s. 20 inch tubes. Obviously MV makes a difference and barrel length affects that.
