Polishing a Shotgun Chamber
Before you choose a method of "polishing" you have to first see what it is you are faced with. WHY are we polishing the chamber? Why do we want to do that?
If it is corrosion, the lighter colored "crystals" in the corrosion are often abrasives. Moving those corrosion crystals under any pressure at all will ETCH the surface of metal. If it is rust, we are dealing with iron oxides that are softer than corrosion crystals, and often if it is light rust, we dont really need to polish, just clean the surface metal. If it is rust in the extreme, then we have several steps to take. If it is propellant residue, then we probably dont want to polish at all, because we simply want to clean the surface. If it is a protectorant, such as preservatives similar to cosmoline, then again, we dont want to polish, only clean.
If you have determined that you are going to "polish" and have your reasons for it, you need to either consider letting your gunsmith handle that task, or consider that in polishing, there is a lip in the farthest reach of the chamber that must remain intact in order for gas pressures in the bore to work uniformly up the barrel. It is a "throat" of sorts, and it's edge and symetry have to be intact.
1. Begin by giving the chamber an extreme cleaning with a common chamber brush and standard solvent suitable for shotgun powder. Get all of the material out of the chamber that you can in this manner. Soak the chamber thoroughly with solvent several times. Let it sit a bit, and brush and swab. Thoroughly clean it in the normal way.
2. See what material is left in the chamber. If there is light pitting, then you might consider leaving it alone, as plastic hulls do well enough in a lightly pitted chamber.
3. Inspect for cracks and faults in the metal that would render the shotgun barrel unsafe. You can to this visually, but remember that the naked eye or even an eye with a magnification tool wont always find a chamber crack. It takes professional equipment, and sometimes Xray to do that. You can find the obvious cracks without equipment. The others - the ones you cant see - are just as dangerous as the obvious ones.
4. Using an oversized chamber mop, attach it to a mandrel of some kind....or perhaps a cleaning rod. Set this into a drill chuck. Clamp the barrell into a bench clamp, protecting the surface of the barrel with either lead vice cap material, or bits of innertube or rag material if you have nothing else. DONT OVERTIGHTEN THE CLAMP - SHOTGUN BARRELS ARE THIN AND OFTEN SOFT.
5. Obtain a block of a compound called "tripoli" which is a buffing compound that comes in bar form. You will find it along side of red, white, and black buffing compound. It is dark brownish red in most cases. Start your drill motor. Hold the buffing compound, and for less than a second each time, "touch" the bar of compound to the spinning over-sized mop, TWICE only. You need almost no compound at all on that bore mop. Traces only.
6. Ensuring you have an oil-free chamber (clean it with a little alcohol on a patch, and then dry it thoroughly) begin buffing the inside of the chamber with the spinning mop. They key to this is to do it lightly to the touch or as was pointed out by one person, you could make "lumpy metal." We dont want to grind, here. We simply want to lightly spin the mop around the surface of the chamber until we get an even looking light-reflective sheen. We are not "honing" here. We dont want to hone, because that is litterally grinding the chamber surface away. We are just lightly polishing the surface, and no farther than that.
7. After polishing, flush the chamber with alcohol, and immediately (before rust can form) clean the chamber and the barrel again with an oil-based bore cleaner such as CLP. Cleaner-Lubricant-Protectorant. Get all traces of tripoli and polishing residues out of that chamber and barrel.
If you put too much tripoli on the bore mop, it will cause too much abrasive to be in the chamber, and it will cause wax-like deposits in places in the chamber that will prevent the compound from evely polishing the surface. This leads to places that are being over-polished and other places that are not polished at all. You only need trace amounts of tripoli. No more than that.
If you are cleaning regularly, you should never have to polish a bore or a chamber. There are shotguns in this world that date back to 1900 that are still showing bright bores and chambers. Cleaning and proper storage are the key.