The worst enemy of gun metal is moisture. While some claim that modern gun powder is less damaging, they ignore the moisture that it attracts and holds inside your gun barrel and other places. There are also some great dry lubricants out there that will protect clean metal surfaces for longer periods than what we had available before they hit the market.
Here's one that I use as a final run through my rifle barrels that has gotten very high marks in test reviews and from buyers alike. I also use it after each trip to the woods to remove any moisture that may have entered my barrel through normal outdoor use.
Hornady One Shot