Depends on your TU. This stuff really only matters to Gearheads like me.
If you want to focus on realism, there are several problems with the ACR. It has to be able to fire two radically different projectiles - a 5 gram HE projectile and a 3 gram DS. it's actually quite a difficult prospect to get a semi- or fully automatic rifle to function properly with ammunition with this kind of variation.
Assuming you solve that problem, to get a long rod, you are probably looking at a projectile closer to 4 or 3mm. Assuming the DS projectiles is some sort of steel alloy, the projectile now ends up with a length/diameter ratio of more like 5:1. of course it's hard to stabilize long rod projectiles with rifling, but that is an entirely different matter.
Another option would be to use a small projectile of high density to increase the relative cross sectional density. This is the reason for using deplete uranium in AP ammunition. DU has the added benfit of being pyrophoric.
But hey, it's your TU. Use what you like.