Irvine's regular guns - not the sniper rifle, which was a one-off - are indeed quite strange.

If you input a command, then try to run from a battle, but [b]then[/i] stop running, Irvine re-loads his gun when he turns around. There, you can clearly see that it's a break-action design, like a shotgun.

It is possible to get guns that fire both conventional ammunition and shot pellets, but they're rare. Doesn't mean that they're unusual in FFVIII's world, though. I like the way Irvine's weapon can hande regular, armour-piercing, flame-throwing, poisonous and shotgun rounds, as well as the WMD-class Pulse Ammo.

Doesn't explain why he can fire so many times in quick succession - as with Quick Shot - without having to reload. Since his gun has a break-action reload, that suggests it has a rather limited capacity...

But then, maybe it takes a magazine, which fits into the space behind the chamber when he reloads?

That'd explain a few things.