I wasn't aware that the Mossbergs did it ever, I would consider it a malfunction. There are a few old pumps that can do this, the Ithaca 37's come to mind. We were thinking about one of those as a defense gun behind the gunshop counter, but in practice it's harder to use than it would seem. Easy to make loud noise, hard to hit anything. I see no reason it wouldn't be legal. Would be the same, function-wise, as "fanning" a single action revolver.