[Unclenick]

I've found the 53 grain MatchKing to be a tack driver in 12" twist, but I run mostly Reloader 10X over 205M primers with it these days. The two 4198's are great accuracy powders in this bullet weight range, but usually sacrifice a little off top velocity.

As far as stability goes, bullet length is more important than weight, and weight is more important than velocity, but all have their effect. Because length is most important, you can shoot the stubbier flat base bullets heavier. The flat base 60, 62, and 64 grain Bergers all have 12" listed as their recommended barrel twist rate. The BC of the 64, is as good as boattail 55's due to its extra weight. Flat bases are usually a little easier to get tight groups with, so, in your shoes, I'd start there.