Are you discussing taking a 6.5 PRC, a 7 PRC or a 300 PRC to a 25 cal? Im assuming a 6.5 PRC. If so those extra 2 inches for a 26" arnt going to give you anything meaningful. With the bulletwieghts and the powder burn rates those extra couple inches arnt going to mean much, I could be wrong. Ive seen people use various powder/bullet combos to get some crazy results.
I guess the question is, what is the rig going to be purpose built for?