Not exactly what you're looking for, but I tried an old Remington #5 Turkey 2 3/4" load in one of our old 870's with the plain jane 28" mod. bbl. and was surprised to see that I'd try it on a turkey at 40 yards if i needed to - couldn't believe the pattern with that load, and that load only! Other loads wouldn't do anywhere near that well. Point is, you don't know till you try it out on the patterning board, and different loads with different guns will pattern differently. Check your velocities, but like mentioned above, the speeds are usually similar with the longer and shorter shells, just the amount of shot is different. With a relatively stationary targets such as varmints, now it's just a matter of how far the patterning board shows you that you can repeatably produce a sure-killing pattern. The same number of pellets at a given speed don't care how many they numbered in the shell! Good hunting, and let us know how you make out at the patterning board - these things interest us all.