I just started using Thurro Clean, and this is a great product.
There are products that do about the same job:
Flitz bore cleaner, liquid Green, no smell
Montana Extreme Copper Cream, no smell, white liquid
Flitz
Regular Grey Jb
I use all of these on badly fouled barrels on punch type jags with patches. If fouling persists, then go to an Iosso or Montana Extreme plastic brush, short stroking back and forth concentrating on problem areas.
In the very worst of cases, I will scrub with a Worn bronze bristle brush with a patch wrapped around it, and also wind Fine Bronze wool in the worn brush and apply the above very, very mild abrasives. This technique never scratches the finest SS barrels as seen under magnification.
The worst of cases get the JB Bore Bright(Red), used on patches with punch type of jag.
I have seen some new cut rifle barrels of late that are just horrible both in the groove and on top of the lands.
Keith Glascow, Winning in the Wind is a champion F Class shooter. He has made some great videos of removing hard carbon on YouTube.
What ever chemicals or application method you use, a Teslong bore scope will teach you to be your own expert.