I disagree with the 'longest shell your shotgun will fire' part for several reasons.
First, the increased recoil of 3" or 3 1/2" shells will slow recovery between shots. Sometimes, there is a need for being able to fire fast- even with a shotgun.
Second, increasing the shell length means less rounds in the magazine in most repeating shotguns since they are usually using a tubular magazine. Yes, you get an increased payload with the longer hull length but I don't see it as a worthy tradeoff. I'd rather have that extra shot or two in the gun- which will more than offset the difference in how many pieces of shot will be sent down range from the total capacity of the gun. Speed loading a tubular magazine is an art unto itself and a very difficult one to master, I'd rather have them in the gun to begin with rather than possibly needing to reload under stress. (Don't believe me? Try it and then watch the 3 gun shooters from the Army Marksmanship Unit over on Utube do it.)
