I had the same problem with the 500 Spl. and the 500 JRH using the RCBS expander die. Notice that the RCBS 500 S&W expander plug is stepped (a noticeable shoulder or ridge). Contacted RCBS and they sent an 50 AE expander, promptly and no charge. The 50 AE expander plug is not stepped, tapers smoothly. Sorry no pictures; however, it solved my and your problem.
Hope this helps,