I purchased a Shilen select match grade SS barrel and had it chambered in 7mm RM. After 2 attempts to break in the barrel it still would copper foul only after 5 shots, so much that the accuracy went from 1/2” too like 3” at 100 yards. And the more copper fouling the larger the group. I did check the speed of the bullet and it was par with what it should be. The copper was only in the last 1/2 of the barrel and not in just in one spot but all the way around. I did try several types of bullets. I returned the barrel to Shilen for inspection. After a couple weeks I received a call from Shilen telling me the barrel was defected, that the bore was over size and they will replace the barrel. My question is why this would cause the barrel to copper foul.
JDY

Blowby of high pressure, hot gas around an ill fitting bullet will result in erosion of metal from the jacket with deposition in the bore.

Jerry Liles