Since the rifle is 20 years old.. I will guess it is a traditional rifle cap lock. Give it a good hot water bath. Use some dish soap in that water. Pay attention to the patches. Are they coming up orange? If they are you have rust in the bore. Keep the patches working the bore. They will start to turn a light brownish orange.

Then get some J-B Bore paste (even if there is not rust). Comet Cleanser will also work, but makes a mess (wet the patch and work the comet into a paste on the patch.. like I said, it is a mess). On a patch, oil the bore first. Then smear a good amount of Paste on a patch and with a tight fitting jag, start working that bore in 2 inch sections at a time. Once you reach bottom repeat the process and work the patch back up. Now change patches, more bore paste, and then full strokes to the bottom and back. Every 15 strokes, switch patches, more bore paste. Do about 50 strokes.

Now give the rifle another hot soap and water bath. After that a brass bore brush and some solvent and scrub the bore real good. Now run solvent patches down the bore. What do they look like? After that some dry patches. Make sure the bore is good and dry.

If you don't have a bore light, a 30-40 caliber empty cartridge casing shined up will work. Drop that down the bore and with a flashlight held to the side, look down the barrel at the bore. Do you see real bad pitting, and ruff spots?

No empty cartridge casings, take a small amount of tin foil. Ball it up smaller then the bore and drop that down the bore. Again with the flash light. It will help you see into the bore. If you have minor damage in the bore, depending on what the rifle is.. let us know and we might be able to recommend a load for it.

If you have any doubt of the integrity of the rifle, do not shoot it.. take it to a gun smith for a professional opinion.