More to the point, like the Mortar. You'll recall that when you "Reload Mortar", the model changes (I can't recall if it's a model change, or an additional proxy). So yeah, magazines sounds like the way to go.
Oh, by the way, the way the weapons are setup, the magazines are typically modelled as part of the weapon (you'll notice that when you reload a weapon, the magazine stays in, while the avatar goes through the motions).