Doug, BP powder has to be pressed and pressed pretty hard. I have read old time hunters used hammer for that (I didn't try that and use my both hands for that). If you have accuracy problems, check all stuff again and again.
When loading BP, there must be enough space for grease cookie and bullet seat. I make grease cookie thickness about 1/8", mix of 50% animal grease and 50% beewax. Without grease cookie using BP loads you just waste your time.
135-140 gr sounds right for 500 BPE and these 5 gr is for barrels regulation.
I tried 3F granulation in 577 BPE even and found it the best for BPE, bullet velocity = to old time catalogs data, but it was Russian BP.
