Zombie thread. It's not a rule buster to revive one, but often a lot of the original participants are no longer active and won't see replies and many are less inclined to chime in when a thread already has a number of posts to read through to get to the modern addition, so it is generally best to start a new thread on the topic.
That's the kit I use. My first Garand came from the now-defunct DCM. Its barrel copper-fouled so badly that I could watch the groups opening up starting at about round 40 of the 50 round National Match Course. The 20-round 600-yard slow fire would start with some 10's and by round 35 I had at least one 9 and at least two more in 35-40, and then in the last 10, there would be at least two 8's and a lot more 9's and maybe a couple of 10's. Then after the match, it would take three or four hours of constant work with Sweet's 7.62 before it finally stopped coming out blue.
When I firelapped it with the NECO kit (using pulled M2 Ball bullets, not cast lapping bullets; the barrel had a long constriction the length of the contour below the lower band seat that needed to be straightened), after the first five coarse rounds it took about 60 passes with Iosso Bore Cleaner (soft abrasive like JB Bore Compound) intermingled with two patches of Butch's Bore Shine to get it clean. At each stage, the number of passes required to get the patches to stop turning blue got fewer and fewer. After the last set of bullets with the 1200 grit final abrasive, it took just ten passes to have no trace. So cleaning got much, much easier and accumulation of copper also stopped. Accuracy was unaffected for the first few rounds and no longer fell apart at higher round counts. Some guns have their accuracy improved by this method, but that element is hit and miss. I know of no instance of it getting worse. The cleaning ease is always greatly improved.
The NECO kit is expensive as these things go, but the laboratory grade abrasive they use works twice as fast and leaves a more uniform surface than you get with technical grades.
Lapping cannot reduce rifling except at the throat, which fire-lapping moves forward a thousandth or two. Once the bullet or lap has rifling marks, it abrades lands and grooves equally thereafter, gradually increasing the gun's caliber, but not reducing the lands preferentially to the grooves and it is very gradual. On the Garand, despite it taking the kit's maximum of 20 rounds of the coarsest abrasive to clear the long contstriction, slugs from the breech end of the barrel were only about 0.0003" larger and still well within the tolerance for nominally .308" groove diameter barrels. The British match teams often used to shoot a lot of .309" groove barrels and claimed to prefer them, though I think they were greasing their bullets. The bottom line, though, is a .308 bullet can easily upset that much with normal firing pressure, so 0.3083" is not going to bother them.
The even application of pressure applies to bullets without abrasive as well. If you have a gun that appears to be shooting its rifling smooth, that almost always turns out to be carbon glaze building up in the grooves. Slip2000 Carbon Killer funneled into the plugged bore and allowed to sit 15 minutes, then allowed to run out into a jar (it's still good) and the bore then brushed will reveal the carbon on its bristles and allow you to work a lot of it out. Repeat until your borescope shows the barrel clean, and you will be amazed to discover the bore color has lightened as well to that of shiny steel and that the rifling has magically re-grown. Carbon Killer is sometimes referred to as rifling fertilizer for this reason.
A slow, but an easier way to do the same thing is to let some Gunzilla run down the barrel and plug it and let it sit for a month or two, then patch it out. The two times I've done this, all the carbon then came out with the first patch and there was no scrubbing left to do. So it's a good approach for seasonal guns that are in their off-season.