ORIGINAL: Triple Se7en
You need a harsh cleaner to remove the grease. You should use rubbing alcohol to remove storing oils. Failure to do both could lead to a sticky bore that appears to be clean - but remains difficult to load a bullet because the bore is tacky.
That may be with the sabots, but not the powerbelts. Out of the correct caliber they are absurdly easy to load even in fouled bores. It sounds to me as though his bore is undersize.