I think it is MORE appropriate for 20" than 16", though the barrel length has nothing to do with effect of twist rate. The reason I say that is that the 20" is more suitable for longer range work and heavier/longer bullets are better for longer ranges. Service rifle competitions shoot out to 600 yds and the 75 gr - 90 gr bullets are great for that. And for longer range having a longer barrel the 20" will give slightly more velocity.
I have a PSA upper with 1:7" twist and it shoots everything from 55 gr to 77 gr very well (1/2" - 7/8" 5-shot groups are easy with handloads).
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