I don't use it in the barrel (another Boretech C4/CU+2 fan here), but it works quite well for cleaning the muzzle of my rifle where the powder and copper gets baked on by the brake. While I don't think it would harm a barrel, I tend to gravitate towards stuff designed for a purpose, all other things being equal.
I usually don't clean the brake proper, but a friend was asking about carbon removal, and how effective CLR is, so I did the following:
This is prior to cleaning. You can see the carbon baked on to the baffle.
View attachment 1095012
I wet a patch with CLR, and stuffed it into the brake so it sit flat on the baffle. After waiting a couple of minutes, I wiped the baffle with the damp patch. This was the result, along with the patch used. [Yeah, I should have included a clean patch as a comparison.]
View attachment 1095013