www.e-gunparts.com
They will have a break down on the 500.
DO NOT take the trigger assy. apart unless you know what your doing!
I'm a smith, and I can tell you most people end up messing them up and need them repaired. You can take the trigger assy out of the receiver, but don't take the assy apart.
For light rust use Rem-Oil and super fine steel wool. Don't use it on the alloy receiver.