Don't be fooled by it's name
Good physics models are hard to come by. So many are out there, and most of them are junk. A good Physics Model is still congruent with the original, yet has minor changes that will change the whole way you play. Good Physics Models don't interfere with winning the game. And they don't make it so easy you are bored after the second level.
One Shot, One Kill is one of the best Physics Models you'll ever see, because it successfully meets all of the above requirements. What does it do, exactly? Simple. It only takes one shot to kill any thing in the game (Except for Juggernauts, which require two shots). The catch is that you can only use your pistol, and one round of ammunition turns into one bullet. Fortunately, each corpse gives away a round, so you'll probably have enough.