Paul, Liam. Pressure is not the main factor in force. It is pressure x surface area on the ram that gives you the force. then the flow (how fast can you get the gas in the ram) determines the speed.

So saying 750psi is too much very much depends on the size of the ram used, and how fast you can fill it. A Dutch robot called Spike uses a FP ram (750-1000 psi) for his axe, but has no more than 1 tonnes of force with his 50mm bore, 150 mm stroke ram. Which is plenty for an axe.