So simple, yet so complete, entertaining, and definitely the most addicting game I've ever played on Kongregate. Usually I cannot stick to one game for any more than 10-15 minutes, but I could play this for hours, I think I will. :)
Just figured it out. You unlock the Tier 2 Assault by beating every challenge, then defeating the assault rifle guy again. You also unlock the Tier 2 Heavy Machine Gun (GAU-19) at level 9. And the pistols at level 10. + if this helped.
 "Autumn War"){kind=icon}
 "Undead Highway"){kind=icon}
 "Monster Slayers"){kind=icon}
 "The Gun Game 2"){kind=icon}
 "Elona Shooter"){kind=icon}