The bottomless ammo Kiriyama had was discussed in Mangafox forums. There people said that probably he got some "help". And he also could take the ammo from the handguns he had taken from the other players, since they are the same caliber size as his uzi.
And yea, the gangsta style is understandable for other students, since they are 15. But Kiriyama was yakuza, if I remember right, so he should have some experience with guns, but he still holds the gun in a weird way.
And, the movie is crappy. I heard the book is best. I have read the manga and watched the movie. The manga is amazing, if we don't count Mitsuko, who totally went crazy.
Aaaaaaaand everyone chooses either BR or HG, why not like both?