To access the cheat menu press the "Z" button at 
the main screen.

The following are the four cheats you can acquire
by getting 2000 or more frags.

All Characters: 
Gives all characters (Not all skins) 

All Weapons: 
Makes the Chestburster selectable without earning 
in Mantid Soldier Trial.

Unlimited Ammo: 
Start with full ammo and never run out 

Extra Modes: 
Opens Time trial and Frag Fest modes without earning 
minigame Icons.