At those velocities, you may find that the rifle will groups even better with a softer alloy, one that will obturate and fill the bore completely. I doubt you are getting that with bought hardcast, as most of that stuff is running at 18-20 BHN and would require approximately 26,000-29,000 psi to obturate. I've begun to work with an alloy that is about 8 BHN in my 357 Max, and it seems to have tightened up my groups, more shooting will of course tell the tale. All that said, if you are not getting any leading, that may be a more important detail to you. Going on to .432 bullets may tighten things up for you as well. A working rule of thumb with cast is to use bulles sized .001-.002" over the groove diameter, this has shown itself to be even more important with the microgroove configuration found on the 357 and 44 barrels.