A gun barrel that is rusty inside can not only affect the weapon's accuracy, it can also negatively affect your safety. The rust and debris within the barrel can cause a drastic increase in pressure after the round is discharged, resulting in a split or bulged barrel--or even exploding the gun's action. Thus, ensuring that the barrel of your rifle, shotgun or pistol is free of rust is of the utmost importance.
Soak the bronze brush in the bore solvent, and attach the brush to your cleaning rod. Run the brush through the rusty barrel. If possible, push the brush in the same direction as the bullet would travel, from the chamber out toward the muzzle. After you've completed several passes through the barrel with the brush, let the gun sit for five minutes.
Attach a clean patch to your cleaning rod, and run it through the barrel. Remove the dirty patch, and attach a new one. Push the new patch through the barrel once again. Keep doing this until a new patch comes out mostly clean.
Soak a large patch in bore solvent. If you do not have different sized patches or can't cut them yourself, you can combine two smaller patches. It is important that the solvent-soaked patch fit snugly inside the gun's barrel.
Attach the solvent-soaked patch to your cleaning rod, and push it through the barrel. Repeat this several times, twisting the rod as you push the patch through the bore to ensure that the entire barrel is coated in solvent. Allow the gun to sit for approximately 10 minutes.
Place a clean patch on your cleaning rod, and push it through the barrel. Once again, remove the dirty patch and replace it with a new one. Run a clean patch through the barrel until it comes out free of any traces of rust or debris.
Soak a clean patch with gun oil, attach it to the cleaning rod and run it through the barrel a few times. This will apply a protective coating to the barrel and help prevent the formation of rust. It is important, however, that you clean the oil out before trying to fire the weapon.
Bore solvents come in various strengths. You should choose both a mild solvent and a more powerful one to use on severely rusted barrels. You can also purchase an actual rust remover from a hardware store and use it to clean the inside of your gun barrel.