That is probably the worst ever advice for removing rust from a barrel.

Never ever use an abrasive with a brush in a barrel.

Use paraffin with a bronze brush. Or paraffin with steel wool. DO NOT USE ABRASIVES WITH A BRUSH!!!

If it is bad enough to use abrasives, use on a tight fitting cloth patch.

Very sad when lovely weapons are neglected and abused

Take a one size smaller nylon brush wrap steel wool around it so it fits tight in the barrel
Put on some automotive valve grinding paste

Run / scrub hard up and down a couple of times
Flush with boiling water
Do the same a couple of times

After last flushing use a phosphor bronze brush with Eosso for a few repetitions
Clean with a 2x4 patch soaked in alcohol until it comes out clean.
Run a sightly wet gun oil patch until that is also clean
Do a visual inspection
Go and shoot
That is what would do

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😮  😮  😮 Nee Fok !!!!! nou weet ek nie. A new barrel and a shot out barrel has a difference of a FEW THOUSANDS of an inch - like lots less than .1 mm and you suggesting grinding paste - steel wool.
Likely the worst possible two things in order mentioned to ever put in a rifled barrel.

That is probably the worst ever advice for removing rust from a barrel.

Never ever use an abrasive with a brush in a barrel.

Use paraffin with a bronze brush. Or paraffin with steel wool. DO NOT USE ABRASIVES WITH A BRUSH!!!

If it is bad enough to use abrasives, use on a tight fitting cloth patch.

I agree with you on the quality of the advice but disagree on your recommendation for using steel wool. A brass scourer is OK being significantly softer than the barrel but I wouldn't use steel. I'd need to have done a lot of brushing before I got to an abrasive of any sort.

I use steelwool on the outside of barrels and bronze brushes inside the barrel.

Fine steel wool and paraffin does not abrade nearly as well as you might think on smooth surfaces. Just use plenty of paraffin. It does help to lift rust from smooth surfaces.

As well as penetrating underneath rust to help lift it, paraffin also acts as a lubricant to avoid scraping the steel. I find it the most reliable way to remove rust from blued surfaces without damaging the surrounds. Just don't go full-retard and use it like sand paper on an old door. Once the surface rust is off, nothing can get rid of the pitting and rough marks.

The problem with a brass scourer is that it is not fine enough for realy fine rust. Probably because fine strands of brass is too soft to do anything usefull.

I use steelwool on the outside of barrels and bronze brushes inside the barrel.

Fine steel wool and paraffin does not abrade nearly as well as you might think on smooth surfaces. Just use plenty of paraffin. It does help to lift rust from smooth surfaces.

As well as penetrating underneath rust to help lift it, paraffin also acts as a lubricant to avoid scraping the steel. I find it the most reliable way to remove rust from blued surfaces without damaging the surrounds. Just don't go full-retard and use it like sand paper on an old door. Once the surface rust is off, nothing can get rid of the pitting and rough marks.

The problem with a brass scourer is that it is not fine enough for realy fine rust. Probably because fine strands of brass is too soft to do anything usefull.

Agree on where steel wool is appropriate for use. I use 0000 steel wool for stock finishing and have used it on really manky steel gun parts. What that's taught me is that it is abrasive enough for me not to want it anywhere near a chamber much less the inside of a barrel. 

Yes 1000 steel wool is brilliant for stocks. Stock oil and s/wool rubbing gives a deep lustre.

BUT ! - the kinda rust I am referring to is not afraid of 1000 steel wool. I think we all talking different degrees here. My method is for built up rust, flake, scarring rust, not surface powder coat rust.

😮  😮  😮 Nee Fok !!!!! nou weet ek nie. A new barrel and a shot out barrel has a difference of a FEW THOUSANDS of an inch - like lots less than .1 mm and you suggesting grinding paste - steel wool.
Likely the worst possible two things in order mentioned to ever put in a rifled barrel.

Do you have any idea how deep the rust has penetrated ?
Once it gets going it NEVER stops
From my reading it appears the barrel has been rusting away for a LONG time
In other words it is ALREADY FUBAR
Steel wool and valve grinding paste are hardly going to make an ALREADY FUBAR barrel much worse
It might even make it a little bit better ...
How much ( and how long ) do you have to grind away to remove the rifling ?

I speak from experience -- getting an old Boer war Lee-Metford that was also neglected for a long time shooting again.

Of course IF the rest of the rifle is in OK condition and is valuable and has sentimental value then ...replace the barrel !

Yes 1000 steel wool is brilliant for stocks. Stock oil and s/wool rubbing gives a deep lustre.

BUT ! - the kinda rust I am referring to is not afraid of 1000 steel wool. I think we all talking different degrees here. My method is for built up rust, flake, scarring rust, not surface powder coat rust.

We are. If I had to take that sort of rust out of a barrel I would have gone through chemical, electrolysis and bronze brush efforts a moer of a lot before I got to steel wool if I ever did and would give up long before I got to something daft like valve grinding paste. Having done a bit of valve grinding in my time I have an appreciation of what it does to hardened valve seats under hand pressure and it has absolutely no place inside a barrel. 

The guy that does Backyard Ballistics on yoochoob has a backround in chemistry and developed a diy evaporust alternative. I made a batch that I used to de-rust some 1950's VW bus parts from my stash that I turned into gun money. It worked incredibly well and is what I would use in a barrel a coupe of hours at a time if a bronze brush ran out of talent.

The Backyard Ballistics rust remover is an affordable, effective DIY chelating solution popular for cleaning rusted metal. It acts as a chelator, removing rust without eating away the underlying metal. The formula, often shared through the "Beyond Ballistics" YouTube channel, is a mixture of water, citric acid, sodium carbonate (or baking soda), and a small amount of dish soap.

Key Components & Recipe (for 1 Liter)
1 Liter Water: Distilled is best to prevent impurities.
100g Citric Acid: Available in powder form online or at many stores.
40g Sodium Carbonate (or 63g Baking Soda): This helps neutralize the acid to a pH of about 4, preventing damage to the underlying metal.
Dish Soap: A small squirt added as a surfactant.

Usage Instructions
Dissolve: Mix the 100g of citric acid into the water until fully dissolved.
Neutralize: Add the 40g of sodium carbonate (washing soda) slowly. The solution will bubble, releasing carbon dioxide.
Surfactant: Add a small amount of dish soap.
Soak: Submerge the rusty item in the solution. For heavy rust, let it soak for a day or longer.
Clean: Remove, scrub if necessary, and rinse thoroughly.
Why It Works
The solution works by chelating iron, creating a solution that targets iron oxide (rust) but leaves the base metal intact, unlike strong acid cleaners. It is reported to be as effective as commercial products like Evapo-Rust for a fraction of the cost

Frank Green (Bartlein barrels) have shown a few pictures of what happens when using abrasive paste with a brush.

The first pass scores tracks as any imperfections of the bore allows. Then for the rest of the barrels short life, the bristles keep following that same path, wearing the score marks deeper until they become gouges, then trenches.

He also showed a few cases of using abrasives with a patch on a regular basis to remove "hard carbon" or just "polish the barrel". Apparently this causes rounded off lands and enlarges the bore. The more aggressive the abrasive, the faster the damage.