The 300S is a 10m match rifle, not really designed to handle that kind of power. Its intended to shoot at 585 fps give or take a bit. The only way to power it up is to put a bigger spring in it, and that will put a lot of stress on the cocking links. Since those links are not designed to handle a heavy spring, they will fail. Additionally, if you were to replace the stock spring with a powerhouse spring you'll be overpowering the piston seal. The 300 doesn't have a regular seal like most spring rifles, but rather has a metal sealing ring like the piston in an internal combustion motor, with a plastic bumper on the front of the piston. Overpowering the piston in this case will cause the piston to slam into the end of the compression chamber, crushing that bumper and eventually damaging the chamber. A good rebuild using stock parts might get you into the 600s, some I have seen close to 650. Other than that stuffing a big spring into one of these guns is not really recommended. JM did a few a long time ago, but he did more than a simple spring swap.
