Really bad idea?

NO!!! Just NO!!! Do not ruin that rifle.

That said, I once obtained an antique early Swiss Vetterli rifle, the first bolt action repeating rifle in general issue, for which there is no ammo available, primarily as it was originally a rimfire cartridge and it was never a common cartridge in the US. The rifle, however, was a relatively uncommon manufacturer and variation of the rifle and had a numbers matched bayonet. Unfortunately, while the rifle was cosmetically clean on the exterior with good blueing and remaining color case hardening on the receiver, as well as great wood, the bore was a sewer pipe. Repeated normal cleaning barely put a dent in it, but at least confirmed it was still rifled.

After speaking to a old collector, I decided to try more aggressive measures and used bronze wool on the jag. A bit better. I then tried steel wool. A bit better, but still dark. Finally, for the big push, I stripped the rifle down to just the barreled action and masked off the muzzle and 6" of the exterior of the barrel and plugged the chamber with a spent cartridge case and silicon sealer. After it dried, I then took a Dewey rod and an old smaller caliber stiff plastic-bristled bore brusn and rapped a small cotton patch over the brush and applied naval jelly to the patch and ran it up and down the bore.

(WARNING! Naval jelly is 70% water and 30% phosphoric acid-based, so you NEED gloves and protective eyewear, etc. It will eat cotton patches and clothes, and totally ruin the finish as blueing is just controlled rust. So be very careful!)

I then took off the brush and patch, cleaned off the rod and put a a clean brush and distilled water-soaked patch on the rod and rinsed out the bore. I did the rinsing with distilled water using new brushes and patches about 5 or 6 times. Each brush was dropped into a baking soda and water mix bowl. I then began using a baking soda water mix to neutralize any remaining acid from the naval jelly. I finished by using boiling water to heat the barrel and after dumping the water, I knocked out the spent cartridge plug with dowel and blew hot air down the bore. The bore now looked acceptable. I then wiped the bore with oil.

The next day I cleaned it with a proper-sized brush and Hoppes. The bore looked much better, had a shine with some less shiny spots, and the rifling was very evident although showed wear, but the rod did not show any noticable rough feel. I repeated the cleaning each day for 2-3 days. I then cleaned and oiled the bore and let it go for 2 days. Upon inspection, and cleaning the patches were clear, I did a thorough cleaning and oiling. After a week I repeated the cleaning and let it sit overnight without oil. The next day there was no evidence of rust so I oiled the bore and reassembled the rifle.

I have considered converting the rifle to centerfire (an easy conversion), but decided to pass as the early versions were made by many makers and are not consistent with bore diameters (they used paper patched bullets) or bolt interchangability and finding a donor bolt that fits is iffy. Modifying the original matching bolt would lower the value. But that old collectable war-horse, the first general-issue bolt action repeating infantry rifle, is restored to shootability.

Yes, very definitely a VERY bad idea! I've never heard of anyone using muriatic acid as a bore cleaner, or for anything else on a firearm. It's certainly not anything you'd want as a first step. The idea is to clean and polish the bore without scratching it or pitting it, leaving a mirror-smooth finish that doesn't collect lead, copper and powder residue. Why on earth would anyone employ a corrosive for that? I've cleaned some extremely neglected 22 bores over the years, and never needed anything more than standard bore cleaners like Hoppe's or perhaps Kroil and J-B Bore Cleaner. I've even tried, briefly, one of the Lyman electrolytic bore cleaners and decided it was way more trouble and effort than it was worse. One of the worst I've seen was another 39A. It was one of the old ones with the Ballard rifling, from before they started using the MicroGroove barrels. It dated from 1947 and had spent it's early years being carried around the Idaho woods by a couple of teenagers. It had a very minor ring in the barrel, right under the rear sight dovetail, from being fired with an obstructed bore at one time or another. The current owner's husband had "cleaned" it for her a few times by dragging a Bore Snake through it. I soaked it overnight with Kroil and then pushed a patch through, bringing out strips of lead several inches long from the grooves. Lots of Kroil, brushing, and general elbow grease involving J-B Bore Cleaner on VFG pellets, and the old girl was shooting 25-yard groups that you could cover with a dime, using iron sights.