The only 50 Cal BPCR experience I have is with 50/70 with a 26 inch twist barrel. That shot the 450 grain Lyman 515141 superbly.
I have much more experience with 45/70 - 45/100. I have had some success with lighter bullets in 16 and 18 twist 45 cal. barrels.
Cast bullet do have an RPM threshold but I would think a 525 grain in a 1/22 twist 50 cal. should be doable based on the 450 in a 26 twist and 45 cal. light bullets in 16 & 18 twist???
Could it be a bullet fit issue or a fouling control issue if you were shooting black?