That is some crud ring if that is what is really happening. I agree with MLKeith, start from scratch and I mean from a very clean barrel. You need to get some solvent. I use Birchwood Casey Bore Scrubber or Butches Bore Shine or M.A.P., but there are others out there. As long as they remove plastic, copper, and lead fowling. Then you need to get a good bore brush and scrub that barrel clean as clean can be....

I am going to say this, but many will disagree...Leave the bore butter out of the barrel. Bore Butter is a great conical and patch lube. Yes I know there are thousands out there that use bore butter with no problems. But you have a problem and it could be the bore butter, then again it might not be, so lets check everything. I personally never had any luck with bore butter as a barrel protector, and I do not lube a barrel to get a sabot down it.. The sabot is wrong for the barrel so you need to look at other sabots..

After you have got that barrel really clean put a high quality gun oil in the barrel. REM OIL or Birchwood Casey Sheath, even Breakfree CLP is a good protector. After you oil up the barrel set it muzzle end down with a paper towel under it for a couple hours and let any excess oil drain back out the muzzle away from the breech.

Before you shoot your super clean rifle barrel take a patch and wet it with isopropyl alcohol. Now swab all the oil out of the barrel. Now a couple dry patches. Leave the last dry patch on the ramrod and push it to the bottom of the breech. Pop a 209 primer. Pull the patch and check it. It should be burnt even damaged. This means the breech is clean and the fire is good.

I am suspecting your problem is in the kind of sabot your shooting. You said you got some shockwaves, so see if they load any better. I can not load a hornady bullet with a harvester sabot down my CVA rifle without almost bending the ramrod. Yet I can load a T/C Mag Express sabot with a hornady bullet down the barrel. It loads stiff but acceptable.

After you shoot, put a patch on the rod, and some 50/50 mixture of alcohol and windshield washer fluid. Work that patch back and fourth in the barrel is short strokes, finally getting all the way down in the breech. If the fowling is real bad, repeat with another wet patch. If the crud ring is bad, use a bore brush and send that down the barrel, then swab again. (I never have to go to that extreme myself). A couple dry patches and you should be able to load a sabot that is loadable.....

Phil the problem I found with inlines is there are some sabots they like and some they hate. It is all a matter of barrel tolorence. Since we do fowl the barrel we have to make sure to clean it between shots in order to get acceptable tolorences back to the sabot. Actually I have one rifle that loads better fowled then clean....

If all else fails turn to conicals. I can load a 460 grain No Excuses Conical down the barrel and shoot five rounds and never have to swab. The accuracy is outstanding, and the down range power is more then you could dream of with 80 grains of Triple Se7en or Goex FFFg.

Keep trying.. that is sure a pretty rifle you have....I saw one on an auction site.