I don't think I will try to search for the entire series although I will ask my buddy about it next time I talk to him on the phone, but I gotta say that those opening credits do have that jazzy panache. The producers for "Baccano!" must have taken a page out of Yoko Kanno's(菅野よう子)"Tank!" and swung for that Big Band swing.
When I saw the title for the opening theme, "Gun's & Roses", I had automatically thought that it was the band Guns & Roses coming up with a rock tune. But as it turned out, it was the title of the song itself, performed by the all-female 9-piece jazz/rock band Paradise Lunch. I couldn't find out a whole lot about them but I recall over 15 years ago that thanks to a 2004 movie called "Swing Girls", there was a brief boom in Big Band jazz (strangely enough, I believe from this decade that it will be a century since The Jazz Age), and maybe Paradise Lunch was one result of the renewed interest; I recall seeing an all-female jazz band taking the stage on TV somewhere when I was still living in Ichikawa.
In any case, "Gun's & Roses", composed by pianist Akane Noguchi(野口茜), is quite the frenzied ride, perhaps along the lines of some of the crazy sequences in "Baccano!" itself (the title means ruckus in Italian) that I've read about. The song weaves in and out between the various players taking on the main theme like me on a tenderloin steak and some good ol' improvisation which is one vital ingredient in the genre. Let's just imagine a Battle of the Bands between Paradise Lunch and the Seatbelts!
There might actually be a Battle of the Themes, though, in my fragile mind. Pianist Makoto Yoshimori(吉森信)was responsible for the soundtrack for "Baccano!", and he came up with "Nagai Nagai Yume no Naka no Utage ~ Theme for 'Baccano!'" (Party in the Long, Long Dream). Sounding like a beefy challenge against "Gun's & Roses", "Nagai Nagai", as it would for a show that mostly takes place during Prohibition-age America, simply reeks of speakeasies and their stench of sweat, stogie smoke, alcohol, gunpowder and occasionally the odd splash of blood. And from around the 3:00 mark, it sounds as if one speakeasy had Eliot Ness and his Untouchables raiding the place with all of the subsequent chaos of gunfire, quick escapes and thrown punches. If the rest of the soundtrack is similar to both themes, then I gather that if I do catch an episode, I will be in for a lot of action on the level of "Indiana Jones" and Brian DePalma's "The Untouchables" with Al Capone's baseball bat reprimand on one unlucky lieutenant being fairly calm in comparison.

