I would suggest 22" on the barrel. I built one last year and love it. My rifle will shoot 1/4-1/2 MOA all day long. I went and shot 60 rounds this am. Here is a silhouette I was shooting at 1012 yards. The 4 shot group at the bottom right is mine. The others were from a 260 that a guy was shooting next to me.
I hand load and use 140 grain Amax bullets in front of 41 grains of H4350. The bullets are running around 2650 fps with this load. You can't push the 140s as hard in a gas gun. I ended up using a JP high pressure bolt and carrier, SLR gas block and a rounded JP ejector to minimize swipes.
These are from a few months ago just short of 1000
100 yard load development