The purpose is "Just another gun" that you don't really need, but have extra cash sitting around burning a hole in your pocket.
You can do whatever you please with it. For me, it would be a beater gun, for fun and fun only. Therefore I want something rugged.
I think i'd want an RPK too.
A rifle for $500? A Mosin and 3,000rd of ammo
I would choose a 30-06. Shoot from 125 grain to 220 grain. Kill anything on North American continent. Get ammo anywhere. IMHO