You could use 'heavy metal' like they use for balancing crankshafts.
"Mallory metal is primarily a tungsten (W) alloy with a little copper (Cu) and nickel (Ni). Tungsten by itself is a very difficult, if not impossilble, to machine. Thus the reason for creating an alloy ("Mallory"). A very dense alloy; much more so than lead. The high cost is due to the tungsten content.
http://www.mallory.com/data/html/1300500.htm"
The other choice would be depleted uranium but you'd have issues with your gun glowing at night.
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