Why Clean Your Barrel?
1. Rust Prevention.The combination of burnt powder, priming compound and metal fouling (especially copper) are ideal ingredients for rust to take hold of in your barrel. And just because you have a stainless steel barrel, don't assume it will not corrode. You just won't see it happening.
2. Accuracy.The build up of copper fouling will decrease accuracy in a centrefire rifle. Excessive leading in your pistol or rifle barrel will also badly affect accuracy, sometimes to the point that bullets will tumble.
3. Safety.Shooting with a badly fouled or rusty barrel could result in excessive pressure in the chamber or barrel, resulting in all sorts of nasties - bulged or split barrels, action blowups, soiled underwear, etc.
4. Recoil.Excessive fouling build-up will increase pressure and therefore felt recoil. Plastic fouling in a shotgun barrel could also conceivably alter the choke characteristics and change your gun's shot pattern.
5. Resale.Even if a pitted bore has not badly affected accuracy (which is possible but not probable), any prospective buyer will have to assume that your rifle is in need of a new barrel. This makes selling it at full value difficult, unless the buyer is optically challenged or downright dumb.
When to Clean Your Barrel?
Airguns. Seldom. Fouling is minimal, but rust can still occur.
Rimfires. With the exception of the 22 Magnum or any other rimfire cartridges using jacketed projectiles, these barrels require little cleaning. Modern 22 ammunition uses lead projectiles which leave traces of lube up the barrel that will not allow the bore to rust. I clean my rimfires seldom, mainly when I know they will be unused for some time.
Centrefire Rifles. Every time they are used, before they are put away. Religiously. Powder and copper fouling must be removed, with an oil coating left inside the bore until it is used again, not forgetting to remove the oil before firing next time.
Centrefire Pistols. Basically the same as for centrefire rifles when using jacketed ammunition. If using lead projectiles, the need to scrub the bore varies depending on the type of projectile, lube and velocity. If leading occurs it should be removed either with a strong bore brush and solvent or lead wipes. I've never been a fan of the old favorite of putting through a few jacketed rounds to clear out the lead.
It never ceases to amaze me how people will make such an issue out of whether ammunition is "corrosive" or not. Some military ammo will cause rust quicker than sporting rounds. But only if you neglect to clean your firearm. Perhaps a bigger issue is whether the projectiles are really copper jacketed, or copper washed mild steel.
How to Clean Your Barrel
Cleaning rod.Preferably one piece, covered. Don't even think about using a pull through.
Bronze brush and jag.I prefer the barber pole style jag to a loop.
Bore solvent. For copper fouled barrels I use two solvents, one mild to get rid of powder fouling, one savage enough to rip the copper out.
Flannelette patches.Precut are convenient, but I prefer to cut my own to get optimum fit in the bore.
A good gun oil,thick enough not to stick on the sides of the bore.
Optional is a bore guide,to center the rod in the barrel and do as little damage to either the crown or the chamber.
The Dirty Deed
Wherever possible push the rod through the barrel in the same direction as the bullet travels. There are a million variations of how many passes with the brush, how many with a patch, which solvent to use, when to mop them out, etc, etc, etc. The following is just a good average.
Pour a little of the mild solvent over the bronze brush. Work up and down the barrel several times. Leave for a couple of minutes. Mop out with a couple of clean patches. This will remove powder fouling.
Take a tight clean patch and pour on some of the stronger solvent. Pass through a couple of times to give a good coating. Leave for 10 minutes. Pass through a couple of clean patches. Repeat until mop-out patches come out with no trace of green/blue copper residue.
Liberally coat a clean patch with oil and pass through a couple of times to give a protective coating to prevent rust. DO NOT FORGET to remove this oil before you shoot the next time. Oil in a barrel constitutes a blockage and will dramatically increase barrel and chamber pressure, possibly resulting in a bulged barrel or worse.
In order to prolong the life of your bronze brush be sure to wash it out in mineral turps or similar to get rid of the solvent.
Resurrecting a Rust Bucket
If your barrel has suffered from past neglect and resembles the Black Hole of Calcutta, you can probably never expect to regain good performance. It is possible, using desperate measures, to make it shootable. None of these methods should be used on anything of any value, and even then should only be used as a last resort.
You have followed the instructions above to clean your rifle. When passing a tight patch through the barrel it feels like you are pushing it against sandpaper. Looking through the barrel the lands and grooves have furry growths over the entire length. Then, and only then, should you consider using these measures.
Start by wrapping fine steel wool around an old bronze brush, using it to scrub the bore with solvent. Should this not have the desired effect, pour boiling water through the bore, cut a tight fitting patch for your jag and apply a small amount of toothpaste to the patch. This is a form of grinding paste and should be used with caution. All traces should be carefully removed when finished. Repeat until satisfied that bore has either improved or never will.