Here's a great tip for you. Once you get the browning (kill 100 red zombies) play "Don't Touch My Gems" and use the browning on that level. That can help a lot to get the "Machine Gun Master" award. I just did that and it jumped from 503 to 856. Hope this helps you.
 "Little Red Riding Hood"){kind=icon}
 "Gun Disassembly"){kind=icon}
 "Insectonator: Zombie mode"){kind=icon}
 "Doodle God 2"){kind=icon}
 "Tripod Attack"){kind=icon}