Have you shot it yet???? and what is heavy???? I can tell you where you can get some excellant 700 gr. bullets but even sub-sonic they would be a killer, on both ends, in a Handy Rifle. I think all of the powder manufacture have data for the 500 S&W on their web page, this is data you can trust. You can also check out www.handloads.org
, but alot of this data was not presure tested. You should have no problems if you stay with bottom end loads and since your looking for sub-sonic. If you need more data PM me and I`ll PM you some.