I'm not a scientist so I can't judge if this is technically possible. But maybe it is a genuine result of a combat fight. However, I know for a fact this has been done to increase the value of less rare medals in the past. You said it yourself in the title "rare medal": The only medals I have seen having a bullet hole are campaign medals (the less scarce ones), bravery medals and combat merit medals... Makes you wonder doesn't it? It would be quite easy to shoot a bullet through a perfectly nice medal to increase its scarceness wouldn't it? ;-)
Btw, I do have some pictures of medals both bullet holes and I will look them up later to show you.