For some reason on AoMr I have a lot of trouble balancing med-packs vs bullets - I often run out of one or the other. my last one was bullets - I killed a captain with my last shot, but a baron incinerated the ammo. You could probably use a few more slots used on medpacks :)
Son of a Gun is ridiculously good. It's what makes pistols worth using, and because of the way the firing time reduction works, each point is better than the last. You should take the 3rd point in it right away, or at level 4 (after Dualgunner), no matter what type of pistol build you want (mods/SoB/Gun Kata). I know that puts off Gun Kata until level 8, but it's worth it.
Visiting Hell's Arena on AoMr can be helpful (you get a more useful reward, instead of the rocket launcher), but it will take some practice before you can do it reliably (and without using lots of medpacks - I still have trouble sometimes). It's a bit easier if it spawns on level 3.
happy hunting!