Mostly dumb luck

That barrel did shoot that load just under 1/4 moa @ 200 for a short time. I can't even do that with a PPC.
Right up until I ran those 26gn VG's at 4700fps. Never would shoot that load or 40gn bullet again. Good lesson learned.
40 Berg is far enough away from the lands in a standard factory Sav that it has left the case long before ever finding them. ;D Shortest bearing length in the 204 family I'm aware of. I've never found best accuracy trying to get close or into the lands with a 204R. They've always preferred a healthy jump for me.
39sbk or 40 V-Max may be an exception to that rule. Seldom use them myself.
The smaller the bore the more fussy they get about fouling. Both carbon and copper. IMO.
NEVER do load development with mixed powders. I have. I've seen the results. The results can leave you chasing your tail forever ;D
FWIW whenever I begin load development with a new powder I do not clean the barrel. This gives me a rough indication of how many rds it will shoot before losing accuracy. With 10X my bet would be 40 - 50 rds and your accuracy will fall apart fast. With 10X You'll need to clean the throat well at that point.