JC - I have no concerns for using an aluminum shim for "proof of concept" when tightening an action. However, as mentioned, wear will be an issue with an aluminum shim, as may battering. As mentioned, the oil and grease used for lube should make a pretty good insulator. Since you are satisfied with the results of aluminum shims, why not change over to a steel shim? Shim stock is available from many machining sources in a wide variety of thicknesses. Match your aluminum shim in steel, super/gorilla glue into place and live happily everafter - OK, maybe not literally everafter, but for a goodly number of rounds through the gun.
