It will have no effect on velocity with modern loads you have got what velocity you are going to get out of a 24" barrel. There are many variables that go into how a given barrel patterns but length is really not one of them, in other words with all else being equal the 24" barrel will pattern as well as the 28" one. Since the barrels are obviously not identical the 24" one may pattern better, you will have to pattern it and see. In my primary 870 the factory IC choke tube patterns better with steel shot than any other tube, including a fairly expensive steel shot specific tube, however in another 870 that I have that same steel shot specific tube patterns best (which made me happy becuase I was thought I had wasted 50 dollars). So you will just have to try the barrel with various choke tubes and see what patterns best with the shotshell you are shooting. Where barrel length does come into play is a longer barrel moves your reference point (bead) farther from the eye which enables some people to shoot better. It also adds a little weight forward of the balance point of the shotgun which helps some people swing smoother and if you have a tendency to stop your swing the inertia of the extra weight helps slightly in overcoming this. Finally the longer barrel results in less precieved noise and blast from the shotgun as it moves the "exhaust port" if you will farther in front of you, this is especially noticeable to folks shooting beside you. However if you like the 28" barrel why not just put camo tape on it or paint it or if your budget permits have it dipped to match your gun and shorter barrel? Good Hunting
