montveil: You're pretty darn close to it. Most birdshot loads aren't loaded to the maximum velocity and pressure that slug and buck loads commonly develop. You don't need maximum velocity to kill a grouse, or even a pheasant. Slug loads are usually maxed out to make them as rifle-like as possible -- lethal out to about 100 yards, much more with the new sabot rounds. So a one-ounce load of #8 shot in a light bird-hunting load will recoil a lot less that a one-ounce slug load that is loaded to the highest velocity that doesn't exceed maximum safe pressure.