@ Weaselthis,..
I got, "Tip TOP" 1/2 MOA, accuracy out of, my 2 Tikka T-3's, one in .270 WSM the other a .243 Win by,..
1) clean, the Barrels with Barnes C-10, to remove existing, copper / fouling.
2) Put a lightly oiled, cotton Patch over, a Bronze Brush "slathered" with, JB's and run this tight fitting, combo, thru your barrel ONE WAY 12-15 strokes ( unscrewing Brush after each pass ). *** Do about, 5-6 Extra "passes" in out of, the THROAT "area" only, do THIS, FIRST ( Smoothes the Reamer marks from, factory Chambering ).
Finish with, the 12-15 passes, ONE Way. Changing Patches, add fresh JB's, a couple of times.
3) Clean barrel and Chamber,.. THOROUGHLY ! Lightly Oil and DRY Barrel and chamber !
4 ) Shoot 5 rounds SLOWLY, Clean, shoot 5 more, Clean again, Barrel should look like Chrome and NOT, "hold" very much Copper, afterward.
My .243 Win Tikka has shot, 1/3", 5 shot groups with, 87 HVLD Berger's and the .270 WSM gets sub 1/2" to 5/8" MOA groups.
The reason your Rifle is showing Copper Fouling is that, the Chamber / throat have been, freshly cut and edges are, Sharp !
I have used, THIS "Method" on 2 Tikka Barrels, A New, 6xc Criterion Barrel and a Green mountain .22 LR Barrel.