Not sure if anyone cares anymore, but you can set up Hermitaur Z with Barrage Piercing to have Cap Up, Bomber, Recoil+1, Reload-1, and Guts if the 2 shot clip annoys you so much. If you're bringing mixes for Normal 2, all you need is a slot for bomberries.
True, between materials, books, and ammo, you'll only have space for 3 extra items, but with guts, that's all you really need (10 potions and 10 Mega Potions, and Cool Drinks/something else for Crimson/White).
I'm not saying Mythical Three-Horn has an overwhelming advantage over Warrior's Bowgun, but dismissing it as useless would be a mistake.
But if this topic is done and nobody cares, then F*** you all.
Warrior's Bowgun has one and only one advantage over Mystical Three Horn: unloading all 3 clusts with absolutely no recoil (with maze's setup that is).
As for the Kirin bowgun... I'd say it's an (more or less, no pierce) universal bowgun that doesn't specialize anything... Opinion might be skewed as I only use LBG for the toughest quests (used them to clear epic quests when I was still a noob conquering G rank, and I cleared HGFFA with Absolute Bow instead of a bow -.-)