Bonanza,
I'm using a 610gr cast bullet from a NEI mould in my 577 3" BPE with good results. My load is 69 gr of IMR 4198, 17 gr of dacron and a mag primer. This load gets me 1750+ fps and regulates well in my rifle. The only hang fires I've had were using large rifle primers, none with the mag primers. If you'd like to try some of these bullets let me know and I'll get some to you.
Best.