I apologize if this has already been posted. If so, delete it.
I've noticed a few things about Irvine's gun that lead me to believe it is some kind of super all-purpose gun. Let's review the facts...
Irvine was integrated into the party as a sniper.
Irvine's limit breaks sometimes involve him shooting off MANY rounds rather quickly.
Irvine's gun most closely resembles a shotgun.
... now, correct me if I'm wrong, but shotguns are crap for long-range and aren't capable of automatic fire...
*realizes most everyone hates Irvine and FFVIII is a game, so it doesn't have to be realistic*
OK, the above is true, but still... couldn't Square have taken the extra effort to changes guns' appearances?