It's a simple answer, but only you can answer. Longer barrels will have more speed, with increased weight and more cumbersome. Shorter barrels are handier, more compact, and less weight. For the 6.5CM, you will gain lose about 15-20 fps per inch. When it comes down to it, the longer barrel won't help you if the shooter can not shoot anyway. I run a 23" on my 6.5x47 with a 140 VLD at 2742 fps. I feel no disadvantage with the slightly slower speeds.