Fire lapping by firing FMJ bullets through a barrel with decreasingly abrasive compounds is not my preferred method for polishing a barrel.
The goal is to smooth microscopically rough surfaces that inevitably occur in the manufacturing process. The same result occurs after firing many hundreds of rounds of jacketed ammo with a fair amount of (sometimes difficult) cleaning.
A smooth, polished bore fouls less, is easier to clean less frequently, and usually produces a slight increase in velocity and more consistent velocities. Polishing typically has no effect on accuracy.
However, some barrels are very poorly machined. The tool that is drawn or pushed through the barrel blank to cut the grooves, leaving the rifling ridges, gets worn. It will "chatter", cut unevenly and leave the barrel unusually rough, like what the OP showed in his photos. QC inspection is supposed to catch when the tool is so worn that it produces such a barrel. Cull the barrel; replace the tool. If a customer gets such a low end barrel, it is easy and relatively cheap just to replace it under warranty.
The barrel may have acceptable accuracy for a defensive pistol, but that accuracy will likely degrade as fouling builds. Pressure can build. Cleaning is very difficult and needs to be done more frequently.
Fire lapping such a barrel is like trying to make a purse out of a sow's ear. The OP said he already used JB Bore Paste, a mildly abrasive cleaner which not only removes heavy fouling but will also polish a clean barrel in a faster, easier, cheaper and more controlled manner than fire lapping, and without firing any ammo.
The amount of "polishing" needed for the OP's barrel to reduce those marks is not worth the time or effort--or the probable result of actually wearing the barrel out. Some serious metal needs to be removed from that bore to get it smoothed and polished. Hence the advice to see if S&W will replace it under warranty.
There are numerous methods available to produce a good bore. Some are cheaper and faster than others, usually reflected in the price of the gun. I suspect these short pistol barrels are usually "button" rifled. Good for price, adequate results when both the tool and the barrels are inspected frequently to ensure the tool made it through the bore properly, or has not become too worn. Every imperfection in the tool is transferred to the barrel.
The roughness shown inside of the OP's barrel would not be acceptable to me. Having used a magnifying bore scope inside many barrels to see the condition of the metal, I can tell from these photos with no magnification that this is an unacceptably rough barrel, whether it is accurate or not. Accuracy is not the only desirable characteristic of a barrel.
