loading dies

[Don McDowell]

You want the expander to be around .001 or .002 above the bullet diameter you're planning to use to allow for spring back of the brass. Buffalo Arms has custom expander plugs, might be worth a call to tell them what you want.
Also if you're just shooting greasers, the Lee 43 mauser die works just right for the 4477. With your RCBS expander die you could then use it as a compression die.