I used two blade mechanicals last year and lost two bucks. One shot was a pass through, the other was not. Also had a doe that took a pass through go way too far. OK, maybe I'm a lousy shot, or just bad luck. I switched to Spitfire mechanical 3 blades this year and am 3 for 3 with two pass throughs and one buried completely inside (hit a little far back at 50 yards and arrow went through the heart, trachea, stopped up inside the neck). None of these 3 got very far. Another bonus is no stupid o-rings to replace. I can't see any reason to use the two blade, O-Ring mechanicals, after trying these three blade Spitfires.