I know you are trying to find a load for the 55 grain btsp's, but I've found that those never group the way the Vmax do, and I have a lot of them too and spfb as I could not pass up the good deals and same here as I did want something inexpensive to shoot with less worry about costs. But with the 1:12 twist I think you will find some 40 grain vmax with a load of the proper H355 could possibly be grouping in the .25's if everything is right. I also did some 35 grainiers, I think Noslers with no lead, and I had a 3/16" group with N133 and the velocity was crazy fast but it has been so long I don't remember what it was. And the worse group on the ladder with those was 1/2". But, they are too expensive to load so it was a one time deal.

But man, that last load you did looks like I may have to go back to the drawing board.