What kind of paint are you using? These cheaper mechanical markers tend to be rougher on paint. The problem could be as simple as using paint that isn't suitable to the marker.
If you chopped paint, then there is more than likely going to be paint in the barrel. That is probably the reason it is so inaccurate. It also wouldn't hurt to look into getting a new barrel. If you match the bore correctly, you will see accuracy go up, along with better efficiency and a quieter sound signature.