ORIGINAL: USMC PMI
What is the purpose? If you are making a short, easy to carry brush gun you may have a drop in FPS but it will not cripple the short range effectivness of any of the calibers listed. Just curious why you would make such a drastic change in barrel length at, as BC stated, the sweet spot.
I am just curious myself since I see rifles offered in these diffrent barrel lengths. My current rifle is a 270 model 70 classic LT with a 24"... Certainly doesn't feel too long for me and I'm very happy with it.