i have never heard of barrel length having anything to do with a guns pattern. i have heard it having alot to do with effective wingshooting.
typically, a shorter barreled gun, like a 24" for example, is utilized as a slug gun or a turkey gun. something where you are aiming and shooting at a stationary target.
"wingshooting" guns..and also competition trap guns, normally have a longer barrel. 28" or even 30". why????
in order to explain this it would take a very looong winded post and maybe even a few diagrams. it has to do with sight picture, swing speed, and the movent of a single point along an arc.
i know a few guys who shoot a 24" gun and do ok with them. alot of it is personal preference..i guess you can get used to anything. personally, i wouldn't let the guy talk you into a 24" gun unless you know exactly what you are getting into. i doubt you'll like it if you haven't used one before.
Friends Don't Let Friends Shoot Mossbergs.