Rock Band 3's achievement "HOPO-cidal Maniac" requires to perform 53,596 hammer-ons and pull-offs total. It can only be done while playing Guitar.
The population of Willamette, Colorado is 53,594. Therefore, if Frank kills this many zombies he has killed the entire population of Willamette
The Apocalypse Frank costume(found under the stairs after obtaining and is exclusive to Chop Till You Drop),obtained after beating the game and killing 53,595 zombies in the process, gives Frank a blonde mowhawk, blue wavy lines across his face,two shoulder pouldrons (the one on the right having spikes and being slightly bigger),laced chest plate,black pants and fur boots. Alternativley you can wear the mowhawk and blue face paint with other clothing items such as the special forces uniform,or prison garb.