Just wondering if going 20" would be to much of a velocity drop. What FPS are you getting with 120-140 class bullets? What is the longest range fired with this set up?
I have a 20" GAP-10 in 6.5CM. I shot some factory 120 amax rounds and it was in the high 2,800's if I remember correctly. The factory 140's are 2,720ish in my rifle.