I just bought a used Mossberg 930 and it appears to have a shim between the synthetic stock and the reciever. The shim looks thinner on the bottom than on the top. The gun pulls up ok but not like the 500's do for me. It points a little, high would the shim be causing this? I'm guessin the stock may be a little short for me and that's why it's pointing slightly high. Can I buy some type of shim kit or shim that will bring it "down" slightly or should I take the shim that's in it out?