I work for the Department of the Army in weapons research. I'm a machinist, toolmaker, QC Inspector and now Materials Technician doing metallographic failure analysis (think CSI gun failure)
That said, every time some yay hoo with more than two stars comes for a visit he hands out "commemorative" coins. They usually have something on the front about his unit and some oorah gung ho lets get 'em motivation on the back. They're only illegal if you use then as legal tender, money.
These coins are forged, as Dennis461 said it ain't easy. Ever seen a die grinder? You know, that 100,000 rpm air motor used to port & polish your heads, it is meant to grind dies. Think of what a die has to do, lots and lots of pressure, extreme heat with accurate repatability. Why do you think the counterfeiters making paper money dies are paid so well?
I know I'm not much help, but it's possible more homework is needed before you jump on this. An accurate cast is much easier to make. Whittle out what you want, make a mold, fill the mold with hot metal and then polish it.
Dan