He should STAY AWAY from the magnum as you can get. You would need a bigger battery anyway. If you think you are going to go through pistons use stock or you will replace your mechbox. Stick with your M100 or M110 an EG1000 will give you plenty of firepower and even then your are jeopardizing your gun. Keep the mechbox simple! If you are trying to get a little boost on ROF try wiring some LARGE capacitors between your battery and motor. this will buffer your power supply and give your motor a more consistent power and raise your ROF. Check out the article at
http://www.airsoftmechanics.com/articles.php?aid=11