I'm thinking really hard.....
it's suprising with how in love with American culture Japan is that there isn't a lot of Western-centric anime.
They'll go as far as telling great gunslinging modern/future action anime (fill in the blanks), gangster flick anime( black lagoon, Gangsta), Supernatural gunslinging (Hellsing, Black Cat, Soul Eater, etc), even mixing medieval fantasy themes with gunslinging like Stephen King (Grenadier. Rushuda Tendo is the greatest pistol user in history not named Vash the Stampede)
But not a lot of flat out western themed anime,
Thats..... strange.
Fist Of The North Star is a big one. I agree though it's a genre that is in only a few anime. Only others that come to mind aside from the ones mentioned here are Space Dandy which is low key and GUN×SWORD.