The answer depends largely on how hot or cold it gets where you live and hunt. You can use the natural materials suggested already. I have a 100 year old Swedish Mauser that came with something akin to waterpump grease in the barrel channel protecting the wood and barrel under the handguard, and the bottom of the barrel from rust. Waxes may melt in heat- some better than others. The same can happen to a heavy waterpump grease. In very cold weather, the wax or grease may get hard and break away from the metal. There are today synthetic based greases, usually silicone, designed by the military for use in Aircraft engines to keep them lubed and running in very cold, Arctic temperatures, That work well in a wide range of temperatures. The nice thing about using something in the barrel channel of the stock is that it protects the barrel from rust, and keeps moisture out of the wood stock. I like to use some stock finish on barrel channels on guns I find have not been finished before they came to me. This seals the wood so water does not penetrate and warp the stock. To be safe, and to protect the barrel from rust, I then use that water pump grease on the bottom of the barrel and in the stock channel. I just remember to keep the gun out direct sunlight, and I don't take it to the range when the temperature reaches 85 degrees and higher. ( I don't enjoy shooting in those conditions, as I easily suffer heat stroke.)
