Although I am not an expert in diagnosing what causes groups to start opening up, I will share what I have found. My 20 Tac and two 223's seem to carbon up and the groups start opening after 30-35 rounds. I read on Sniper Hide, that those shooters felt carbon was usually the first sign of groups getting larger. In fact, some of them think it is worse than copper.
Anyway, I find if I use Brake Cleaner after about 20-30 rounds I can hold my group size by removing the carbon. I have found on my Remington 204, PacNor barreled 20 tac and 223's do not need to be treated for copper until I have 100-150 rounds thru them. After reading some of the horror stories about coppering, I feel fortunate. I am curious if some of you have like experiences similar.
