Which is more a set of combat rules than rpg rules. It is the small arms combat system and covers small arms ok. The game scale is 2 seconds a Phase, which is broken up into .5 second impulses and 2 yards a hex or inch. Each character has a number of CA or combat actions with which to act. CA are also used for aiming.
High pen does all the killing.
Which is basically how big a hole they tear through flesh. The bigger the DC the deader you become. I've had a situation were a face plate of 4 PF has saved a character from having his brains blown out by a .45 to just his jaw blown off. All the frag, shotgun and auto fire rules use geometry to calculate how many bits hit a target. This may change with range firing stance etc.