Well it depends on how you define a "shotgun" effect. The Luna can kill in one hit, but it's pretty tough to depend on that with the extremely short range. The basic Luna strategy is to dominate at close range with two hits, such that opponents must flee as soon as they're tagged or get wrecked. Its specialty is Tower Control.What do you guys think of the Lunar Blaster Neo? I thought i was dead set on longer range blasters like the N-Zap or the Splattershots now but then this guy absolutely annihilated me just now with a Lunar Blaster Neo so i've been rethinking the whole thing... It's sort of like a shotgun at close range right?
That said, there are other close range weapons that can also lay claim to the shotgun effect. Rollers, in particularly the carbon, are more reliable for one hit KOs. The normal blaster has a bit more range than the Luna and can be more reliable for one hit KOs if you can aim, while retaining solid area control abilities with two hit KOs. And my baby, the tri-slosher, requires two hits but has great spread and firing rate.
The flavor variety is one of the things that makes Splatoon so great. And I concur that swim speed is really helpful for close range weapons, for approaching, flanking, and retreating.