I've built quite a few guns like this. Application is what dictates barrel length. If you are running in say, a 500 yard "kill box", then you can do it with an 18" barrel. If your attempting to run past that, even the most efficient bullets loose some steam and velocity becomes a whole lot more relevant. This is where the 24"+ length starts to work better. I've never had a client approach me about going beyond 26" on a 6mm for a hunting app. We do them for square range competitive rifles all the time. Totally different application though.
If your taking the gun from truck to hill top, run a long stick and stretch your legs with it. Crawling around or road hunting coyotes from a truck window will command a shorter barrel.
C.
