Some guns and some people don't like 777. You need a different sabot from what you are saying if a gun loads too tight it won't shoot good, also don't go over board if it loads too loose it won't shoot good, there is a certain narrow area where sabots shoot good. As mentioned you have to swab every shot with 777 and if you get a bad crud ring then swabbing can be quite a chore but to get accuracy with 777 in most guns the barrel must be clean of most crud and any crud rings.
You might check out that gun with the ribbed sabot Harvester sells they are for tight barrels and I would suggest a improved Blackhorn breach plug and a trial can of
Blackhorn.