People changing buffer weights and springs are trying to fine tune or fix a problem with their rifle. Like mentioned, spring and buffer are dependent on the stock because of the length of travel for the buffer to absorb recoil and cycle the weapon, get the buffer and spring for the type of stock you have, it should work with any barrel, gas system combination.
Correct me if I'm wrong, but I believe the longer Rifle stock buffer system should supply a softer recoil, while the shorter carbine collapsible stock buffer system should be a little more harsh. The gas system should effect the recoil more than the buffer system. If you have a mid-lenth gas system, a normal collapsible stock buffer system should work just fine and have a very reasonable recoil.