Young Gun Carnaval is another manga based off a light novel. It only covers half of the novel and was then finished, a bit prematurely. Fans in Japan also complained that it was nowhere near as good as the novel and that Yumika seemed to have been less well done as a character. In fact, the manga although trying to give time to both characters, seems to emphasize Jinpachi, which in this case is a good thing, since he was better done.
That being said, the 5 volume manga has a fair amount of realism for a comedy/action and the art isn't bad at all, in my opinion, though it is no masterpiece. The author has done some research into his gun models and all of the organizations and illegal business seem relatively believable.
I recommend this for a fast paced read, it is definitely enjoyable, particularly if you like action flick type series like sun-ken rock or dendrobates.
Note: Although it is a seinen and was published in a seinen magazine, I personally think this is more like a shounen. It is has a realistic tone, which makes it borderline between the two genres, however.