With your short draw length and poundage IMO you should stay away from mechanicals. Not sure what speed you're getting but it sounds like you don't have the KE to get a pass through with a big cut mechanical. Get a cut on contact style broadhead for deeper penetration and avoid hitting the shoulder. A good strong COC head like the Slick Trick Viper Trick is a stong broadhead and will shoot along side your field tips if your bow is properly tuned. If they don't read the threads about bow tuning or the Easton bow tuning guide.