I think you may have some plastic fouling also causing your problem. You need to really clean the barrel. I would suggest (some will not agree so you decide) trying to clean the bore with a solvent for plastic. After that and you are pretty sure the barrel is clean. I am guessing that you have only shot sabot bullet combinations. You may have some initial rough portions in the barrel from initial machining that could cause the plastic fouling to be worse than it usually is so; here is the part that some people will not agree with. I had similar problems but mostly with 777 fouling in my rifle. I got some JB bore paste and put a brass brush on a heavy cleaning rod put a patch on the brush and load it with the paste. Make several full length passes up and down the barrel. Do about 25 to 30 up and down and reload the patch with paste. Keep this up for at least 100 passes. Change patches as the patch looks bad and after all this re-clean the barrel with a strong solvent dry and either oil for protection if you can't shoot right away or try loading and shooting again.