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http://monsterhunter.wikia.com/wiki/MH3:_Bowgun#Heavy_Bowgun

Towards the bottom of each weapon weight they give a combination of different pieces. I'm not much of a gunner and don't really have much experience with them, but from what I can gather these are decent combination of pieces.

Just look up "Monster Hunter Tri Bowgun guide" and there should be this one by VampCosmaut (I bet i spelt it wrong) or something like that. It's extremely useful.
It depends what you're looking for in a bowgun. If you want a good all around bowgun then I'd say Thundacrus rex frame, Devil's grin barrel, and Chaos wing stock. That would go well with the full Vangis armor.
In terms of pure damage, I'd go with a pierce armor set and
a bowgun consisting of a Thund rex frame, devil's grin barrel and thundacrus stock Oh and the pierce armor set is Diablos+. Since that's only 3 armor pieces, mix it up with Barioth+ or something to give you evade if you want.