I bought the 6,5CM last summer. Great rifle, I was meaning.....
Mostly I shoot 10 or 15 rounds for training. With excelent groups. But at competion, mostly with 25 rounds or more, the problem was coming...
After 15 rounds the group is moving down and left. Not much, nearly one inch...
First I was meaning it´s the scope and I bought a new one. But today after the 15th round, same problem.....
Anyone an idea?

Is it because of fouling or the barrel heating up?

I mean,it´s the heating.

I have never shot competition so my advise my be null and void, but I have been at the range shooting different loads to see what a rifle liked for accuracy and getting impatient for the barrel to cool would pour cool water down the tube. If this is feasible to do it will cool a barrel quickly, but keep in mind I have never heated a barrel extremely hot and I can imagine getting adverse results from this cooling method on a super hot barrel.

Also, some propellants will heat a barrel faster than others and have found a few that I refuse to burn for this reason.
A fluted heavy varminter barrel is hard for me to heat up. I have shot twenty rounds through my 22/250Rem that wears an E.R. Shaw helical fluted barrel without getting warm enough to quit shooting, and in fact I couldn't hardly feel much warmth at all with my hand.