To allow the paintball to be accelerated to 300fps in the required distance takes a certain pressure behind the ball. The mag has about 60 psi behind the ball even though the chamber pressure is at ~400psi. This is due to restrictions in the flow path such as the size of the power tube and the bolt stem being in the way. The ball could be accelerated to 300fps in the same distance using a lower chamber pressure if the air could be delivered with less restriction. Ultimately we would require the same 60 psi behind the ball pressure starting out.
All that being said, the mag's biggest problem is not the actual operating pressure but the rate of acceleration on the bolt. This causes the bolt to strike the ball with a lot of force and could cause breakage if using bad or brittle paint. We need a two stage pressure system. First stage gently accelerates the bolt to strike the ball and the second stage releases the full force of the chamber pressure (we still want about 60 psi behind the ball).
Now looking at what bjjb99 said, it appears that there isn't much of an opening for the air to pass out by the bolt stem from the power tube. If we are restricting the air flow anyway, why not do it in two stages. Start the power tube at a smaller diameter at the back, sealed with an o-ring, and then open it up to the wider opening near the front. This would cause the force on the bolt to be lessened (the total force on the bolt is directly related to the cross sectional area of the object and the psi behind it) for the initial push. When the bolt reached a forward point it would open up to the larger tube diameter and by now the smaller part of the tube would be free of any obstruction and the pressure would be released to the ball much like it is now at about 60 psi. We wouldn't have to restrict the back of the power tube very much either. If we reduced the total cross sectional area by half it would effectively reduce the force on the bolt by half as well. The power tube would still be fairly large so the majority of the restriction would still be at the front where it is now.
I have to think about this some more. Keep the ideas flowing guys.
Except for the Automag in front, its usually the man behind the equipment that counts.