If you don't want to hurt your shoulder, don't shoot the gun on your shoulder! That's a rotator cuff injury waiting to happen. Leverage, my friend.
Shoot a shotgun by seating the butt on your upper chest, NEVER out on your shoulder, out past the joint. Do that enough, and you WILL hurt your shoulder, and no buttpad will help that, since you're dealing with force that can hurt your joint, not just impact that will cause a superficial bruise.
Now, in order for that not to slam your clavicle (the little bone that appears as a ridge running across between your shoulder and your neck), the Limbsaver is a great help, in my experience. It will pad the butt so that it won't hurt when the gun recoils against your clavicle, which happens sometimes when you use a proper chest hold. Mine is a godsend. (With a field gun like a Nova, which has more drop in the stock, this is less of a big deal, but I have my limbsaver on a trap gun, which has a straight comb, so it sits a tad higher. Still, I think it's worth the $30.)
And agreed with BT on the Limbsaver being superior to other buttpads, for this purpose. It's really thick and soft, yet it doesn't collapse, it's more bouncy.
-Barry (shotgun addict and CSCS)