Nothing goes in the gas tube. Putting anything down it leave the possibility of it breaking off in the tube and causing far more problems.

In short leave it be.

For copper, hoppes #9 is a great all around cleaner with the exception of copper there are much better copper solvents. Sweets 762 is pretty strong and I've had good luck with wipe out patch out which also wont kill you with fumes. Ammonia is what eats/dissolves the copper, just don't leave it sitting in a barrel. 10 minutes should be plenty of time, then chase with a dry patch followed by oil and another dry patch to get excess oil out. When you leave ammonia products sitting in your barrel you run into problems, this goes for all barrels.