I have tried various lengths and contours in the 300-500 yard range. My all out favorite as far as accuracy and consistency, cold bore/hot bore, is my DPMS 24" varmint barrel upper with a 1:9 twist. This rifle is awfully heavy and bulky but will hold .25 MOA consistantly using 69gr SMKs and I have not had to re zero for the last 2100 rounds. I have a couple of H Bar barreled uppers in 16- 20" lengths also 1:9 twist but they run .75-1 MOA and require zero checks and adjustments for best results at the longer distances. Interestingly, I have a Remington R-15 Varmint with the 26" triangular barrel in 1:9, 24" barrel that performs just about as well as the DPMS, but is a lot lighter and more manageable for carrying when coyote hunting
. I was very skeptical about this rifle at first, but once I put a decent trigger in it, I found it to be my preferred long range carry rifle for coyote hunting.