I shoot 10gr of Unique with 240gr Semiwadcutter Trushot cast. Thats my accuracy load. I was told by oregon trail that I could go up to 11.6gr of Unique. They sent me a loading guide once which had the midrange powders like HS-6 in it. I will have to get the info when I go home. But I have tried HP-38 and know its close to unique for plinking. And for 240gr you can go up to 11gr. I wouldn't hesitate to go up to 10gr of HP-38 with 250gr cast.

I know that doesn't exactly answer your question. I am just not used to shooting 250gr in 44mag.