Right remember the Reckoners accelerated Mil-Tech by about a two tech levels, so in the late 2080's we have large laser guns, plasma blades, and anti-missle capabilites. So anti-armour rounds for standard guns should not be that uncommon. Yet also remember that most of what survived the war is not the hard core military stuff, but the stuff that was cached, I.E. National Gaurd armories.