Carbon build up??

Whats a recommended lapping paste, some fine stuff Id imagine? 600 grit? Tried the 220 lapping paste and brass screw trick to recrown before but thats too much no?

I have a question, would using a small square of cloth with coconut oil (the kind you can buy in gun stores) on it running it through the bore hurt anything? I know it removes bluing so if you did that you would have to be extremely careful to only touch the bore but seems to me it would work removing carbon buildup. I have a Colt King Cobra revolver from the '80s that is stainless steel and I use the coconut cloth to remove the powder and burn marks off the cylinder face all the time and it works like a charm.but I have never heard of anyone using it in the bore of a barrel and there may be a reason for that. Just figured I would bring it up as a possible solution?

any places raised from surface rust can be taken care of with JB paste and elbow grease. I would start there, then shoot it before attempting anything more agressive.

Put a cork in the muzzle, fill the barrel with regular bore solvent and leave it for a few hours. It won't fix or even help with the corrosion and pitting but the carbon will be loosened.
Endless patches and brushes won't do anything.

I used jb bore paste going at it for a couple hours to nominal if any result and I have plugged the barrel and let solvent soak for a couple days twice, that did seem to help a bit but scrubbing with brushes and patches seem to be necessary for a while

You can also pour boiling water down from the chamber (Rig up a plastic tube and funnel thats what I did)end patch out whilst hot let it cool the excessive heat will dry any moisture then when you can just barely rest your hand on it get to work with either JB paste and Sweets there are two types of JB paste you'll want the Greeny/Grey looking one the Red one is super fine for polishing bores on F-class rigs.
Don't leave the sweets in too long its pretty wicked stuff after you have patched it swab the bore with methylated spirits let dry then oil it up with sweets oil as that will continue to work on the copper wash your bronze brushes in metho after wards otherwise the sweets will still work on the copper in them.
Have plenty of patches and knock yourself up a bore guide from the chamber end if working from the muzzle be very careful you don't induce the symptom called cord wear on one side of the muzzle be prepared for plenty of elbow exercise don't cut your 4x2 too wide to start with either the rough bore is gunna grab onto the flannelette like a virgins draws at midnight.

Much of what people refer to as "Carbon" is oxidized detritus from the bullet jacket and some from the actual case. Smokeless propellants leave very little residue because the Nitro-Cellulose (and Nitro-Glycerine) are compounds, not "mixtures" like black powder and thus, pretty much the entire propellant goes from solid to gas rather rapidly. About the only "Carbon" that may be in the propellant, apart from that in the "Cellulose" part of the "Nitro-Cellulose", is a tiny amount of Graphite used as a static electricity "suppressant" and "flow improver" in the loading process. Any stray "Carbon" in the bore after ignition will probably bond with the atmospheric oxygen in the bore at the sort of temperatures involved. There is, however, that other "greenhouse gas", water vapour, formed in all this high-temperature conversion from solid to gas. Not likely to stick around long in a warm barrel, however.

Think about the forces and temperatures at work when a cartridge fires and it makes sense.

Nickel bullet jackets certainly leave nasty "streaks" in barrels. Gilding metal (basically a soft brass) also leaves distinctly-coloured streaks. Leave that lot sit in a barrel that is regularly firing corrosively primed ammo, i.e. almost ALL surplus .303 ammo ever made, and the chloride salts from primer ignition will be embedded in layers in your bore. Hence the time-honoured trick with the boiling water and the nifty funnel.

Most modern bore cleaners will work, but it's not "a quick once-over" and done. If you are using "surplus" ammo, re-clean a couple of days later. (Also done with the "water-cleaned" rifles). The crud that "sweats" out of the bore over a couple of days is disturbing when first encountered.

Even chrome-lined barrels are subject to corrosion with ANY ammo, if not given correct attention.

The chrome lining is not some impervious miracle shield. It is basically there to reduce wear and erosion, not totally remove the need for cleaning. Hard chroming in barrels is utterly different from what is found on the "shiny bits" of classic cars. There is NO pre-plating with copper, for a start. Hard Chrome is formed on the steel substrate as tiny "nodules", and it actually looks "lumpy' under a microscope. However, this is actually part of the reason chrome lined barrels seem to be "slick" to bullets. The bullet is actually riding on the tops of these little nodules and thus has less surface contact and thus "drag". Back in my younger days of shooting the L1A1 in service and personal use, there was a quick way to tell if the barrel had a fully chromed chamber and bore, or, as in early ones, just a plated chamber. Run a loose patch, soaked in "Sweets" solvent through the barrel. Wait about a minute or so and then run a firm-fitting, clean dry patch through. The "standard" bore would show distinct blue streaks, indicating the removal of jacket fouling. The fully-chromed bores showed virtually NO colouration for the same amount of rounds fired.

The downside of this "slick lumpiness" is that there is micro-porosity between these nodules and that is where the corrosive molecules find their way to the steel substrate. The result is RUST underneath the chrome and the first thing people start to notice is "chunks' of chrome going missing, especially near the muzzle and back near the throat / leade. Furthermore, chrome lining works fairly poorly on conventional "square-cut" rifling. Sharp, external corners, such as on each side of a the lands, create a higher current density and thus greater chrome deposition. This tends to be fragile and gets bashed off by bullets traveling up the bore. The reverse occurs in the tiny corners at the bottom od the grooves, Poor deposition leads to greater risk of corrosion. That is why well-designed chrome-lined barrels have radiused corners on their rifling; see H&K style "polygonal" rifling for the idea. Ditto "5R' rifling.