You could try some Pyro RS. It will shoot softer than the P.

I assume you're talking about shooting a PRB?

As a last resort you can cut off the curved butt, and install a recoil pad. (Sims)

I have a feeling you're shouldering the gun wrong, but I have no way of knowing without seeing you do it. Your load with the weight of a GPR shouldn't kick bad at all. Your body should be almost 90 degrees to the target with the gun going across your body.

Are you shooting offhand? Shooting off a bench will kill you with that gun. It did me no matter how I mounted it, but offhand it was fine.