You say you have less that 2000 rounds through the barrel, so I am guessing you are close in round count to 2000? At 2000 rounds the bearrel should be well broken in. I am not a big believer in abrasive bullets like what Tubb sells, and I would say that in your case would not be something I would do.

My experience with the 204 in the past I have found that this round is one that likes to carbon foul. I found that once the carbon fouled they started to keyhole, and would continue to do so until you got the carbon out. (On of the reasons I quit shooting the 204) Back then (5 or more years ago) the bronze brushes in 20 cal was junk, they lasted about three strokes and they was done. The only 20 Cal brush I have ever found that was worth a dang was the Tipton brushes. I found the best product for heavy carbon fouling is JB bore cleaner. How are you presently removing carbon build up?

I would get a bore solvent that works on carbon, and not a universal or cooper cleaning solvent. I love GM TEC but it is no longer available, you may be able to find Mercury Power tune, at a boat Marina it is the same stuff. I would take something like work it with a brush and then let it sit in the bore overnight, then follow up with a scrubbing of JB, then a couple of patches of bore cleaner, and then patch it all out. Some people have reported good luck with KG-1 carbon remover, I can not say I have tried it. http://www.midwayusa.com/viewProduct...tNumber=470468 Some people claim seafoam is good, I can say I never had good luck with it, It is no GM TEC.

I am willing to bet once you get the copper out it will stop keyholing. I will say one thing 2000 rounds is a high round count on any barrel, you may just want to think about re-barreling it.