Correct, thanks, here is a post by Jason showing more viewtopic.php?f=8&t=12824. What makes it so heavy?
The tungsten carbide.
Definitely, a US 105MM M735 sub-projectile dart. This is a tactical round with a heavy tungsten core like EOD mentioned. It was originally developed for the M1 Abrams tank fitted with the M68 105MM gun system. It was replaced briefly by the M774, then the M833 and finally the M900, all DU cored rounds before the US upgraded to the 120MM M1A1 Abrams tank. The correct case for it is a steel M148A1B1.
PS: Nice score, Vlad :-)