My .300 WSM
seems to copper foul very significantly with Accubonds, just a function of that particular barrel because I do not see it in other rifles. Probably also the higher velocities involved, since most of my Accubond shooting has been with .308 Win.
Strangely the Accubond CT Winchester factory load does not seem to copper foul, can shoot two or three boxes with no accuracy falloff.
I believe what happened to me was I did not recognize the copper fouling buildup and was not addressing it with my cleaning procedure. Now I have a new solvent that seems to get it out faster and more completely, no wash or anything in the bore after a good cleaning.
I believe that most of my magnums copper foul significantly more than my non-magnums, might be the price we pay for higher velocities.
No answer from Winchester re your .300WSM Stealth, will pass on the info when I get it.
I also found that overall length had a lot to do with my WSM accuracy,