T/C 13 Bore Cleaner works fine with leaded conicals but you need something stronger for sabots. Your keyholing problem is due to sabot plastic filling your rifling.

Go to Wally World & either buy that spray can of Outers Nitro Solvent they have or that tube? of Remington Bore Cleaner. Wet your first patch & run it up & down the bore a few times. Let it work for about one hour with your bore to the ground with a rag under the muzzle. Repeated treatments with a bore brush (not a jag) will remove that plastic.

Always run two nitro patches & bore brush after range work while using sabots, copper... etc. I' ll bet the next you go to the range, your bullets won' t keyhole & your 6" Powerbelt groups will shrink to 2-3" .