In order to continue working properly, all guns need cleaning. That applies even to the famously rugged AK-47, although that gun is far more forgiving of neglect than most. Gunpowder is a dirty thing, and leaves behind burnt residue when it is used. Over time, this will foul the working parts of any firearm. Also, those moving parts require lubrication, just like the moving parts of any machine.
Gun cleaning always starts by checking the firearm to make sure it is unloaded. This is a safety-measure that is overlooked only by the foolish and reckless, especially since disassembly of most firearms requires pulling the trigger to release the weapon’s action.
After this, the procedure usually requires disassembly of the gun. Breech-loaders such as the break-open action of a double barrel shotgun will require no disassembly, but almost all other firearms will. How this is done therefore obviously depends on the gun in question, and the weapon’s manual should be consulted for specific instructions. However, an semi-complicated example is the .30-30 lever-action rifle, a popular firearm among hunters.
As previously noted, start by pulling the trigger and releasing the firing spring. Then push the lever forward, and take out the hinge screw. This will allow the bolt and the lever to be pulled out of the rifle. These parts should then be either soaked in gun cleaning solvent, or liberally coated with solvent using a brush. The chamber also needs cleaning with the brush and solvent. The barrel should be cleaned using a ramrod mounting a solvent-soaked pad. An old toothbrush can be used to remove stubborn, scorched powder residue. With the grime and powder cleaned out, the parts should be wiped down. Finally, apply gun oil to the springs. The gun is now clean and ready for assembly.
Substances other than gun solvent are often used as cheap substitutes, such as carburetor cleaner or diesel fuel. Many old soldier’s yarns tell of dunking entire firearms into a drum of diesel, leaving it for several minutes, and then carrying the gun off into the field ready to go. Diesel fuel does break down gunpowder residue, and the fuel itself needs to be pressurized to ignite, and therefore won’t burn in the chamber of a gun. However, the truth is that gun solvent does a better job than diesel, and unless a gun-owner is in the military, it is doubtful they have a bucket or drum of diesel just laying around to use for gun cleaning. There is also the question of what to do with a drum of diesel that will eventually become fouled with burnt gunpowder.
Another cheap substance that is sometimes used is carburetor cleaner, and this should be avoided. It also breaks down gunpowder, but unlike diesel it can also break down the finish of a fine firearm. Gun solvent is based on nitrates, while carburetor cleaner uses toluene-based solvents
Sources: http://www.sharecorp.com/msds/8310CarbCleaner.pdf; http://www.cabelas.com/ssubcat-1/cat20797.shtml; http://www.popularmechanics.com/technology/upgrade/4223337.html?page=19.