Just to add to what the others have said, I am a big fan of leather slip-on pads precisely to avoid altering a factory original stock (I always seem to need a little more LOP, and a gunsmith always wants to cut flat the curved butt in order to add an extended buttpad).
I shoot my Winchester 101 12ga from low-gun all summer long with a leather slip-on pad at sporting clays. The pad gets so much use the leather gets a "polished" surface and becomes real slippery. About once a month I rough up the leather surface with some sand paper so it has the right amount of "gription."
I've always been very satisfied with the inexpensive Galco pads. I once tried out a high end pad made of very heavy leather. The pad was beautifully made, but the heavy, stiff leather interfered with the pad fully conforming to the shape of the butt, and I could never get the pad tight enough to not move under recoil. The less expensive Galco pads have thinner, softer leather which better conforms to the butt when the pad is snugged down, and I never have movement problems with such pads.
Also, I've heard that the tanning chemicals in the leather may discolor the wood finish if a leather pad is left on a gun too long. I imagine that this would take at least many months to happen, but I always remove the pad after each shooting session (in case I happen to not shoot the gun for a long time). Consequently, I prefer the quick-to-attach-and-detach velcro-type leather slip-on pads over the lace-on type.
--shinbone
