The best 'system' I can think of is the dice=rolling system.

Everyone starts with low stats of Vitality, Defense, and Aim.  Weapons and armor increase these stats.  Elevation and Cover increases these stats.

When you attack, you roll the d20 and add that value to either Defense or Aim, add modifiers, then compare it to the opponents own value of the d20+Aim/Defense.  If the attacker succeeds, he deals as much damage as his weapon allows.  If he fails, the shot misses.

EDIT:  I'll get on an OOC I reckon.