...................... Overall it seems that the 1:18 is preferred, and that’s what is winning competitions. My question is if bullet length is relative to the twist rate then why wouldn’t a 1:20 twist with the correct length bullet shoot as good as the 1:18 with the correct length bullet at say 1000 yards? The 1:18 stabilizes a longer bullet better than 1:20, so are the longer bullets in general better for long range?
Only considering 45cal..............
There's an awful lot involved in LONG RANGE shooting, especially 1,000yds. To my knowledge, there's only 4 of us on this site shooting MODERN INLINES to 1,000yds. Quite frankly, all 4 of us are still learning.
The potential problems with a 1:20, even shooting a great bullet for that twist rate, would be yes, length and its BC. Heavier and longer bullets are the rule. 275gr and 300gr bullets are very poor choices for shooting 1,000yds. Bullets of at least 325grs with very high BC's, even better at 350grs, work well at 1,000yds out of a 1:19 twist and the 1:18's.
Propellant charges in competitions is limited to 120grs VOLUME of BH209, giving approximate velocities of around 2,180fps +/-. Bullets will go sub-sonic at approximately 800yds.
This Arrowhead 350gr XLD has won 2 of the 3 long range matches of 800, 900 and 1,000yds.
Although a quality bullet, the Pittman Accumax 275gr bullet, it is not a LONG range bullet and wouldn't be used in competition.
This is a Fury 300gr. It is an outstanding hunting bullet but, it doesn't stabilize well beyond 400yds.