Stick with a short action cartridge. Such as the .308, 7mm-08, .260, .243, etc... etc...
Out of a 15" pistol barrel you aren't going to gain squat with more powder, aside from muzzle blast and a big ball of flame. A 7mm-08 will fire a 140 grain bullet equally as fast as the 25-06 will fire a 117 grain bullet out of a 15" barrel. Which should be about 2500 - 2600 fps.