In
this thread, the weapon discussed as a BFTG was a .50 Alaskan firing a 450gr bullet at 1,905fps for 3,627ft-lbs of kinetic energy at the muzzle. This was, of course, for a handgun and not a shoulder arm, but the Rheinmetall 120mm M256 smoothbore cannon might still be a bit much: firing the M829A2 APFSDS-T round, the 73,233gr (10.45lbs) depleted uranium penetrator leaves the muzzle at around 5,830fps, which comes to 5,528,405ft-lbs of kinetic energy.
I haven't got any data on the exact ballistics of the XM1028 APERS round, but it's a safe bet it's somewhere in the 4.5 - 6.5 million ft-lbs range. Basically, it fires the equivalent of 130-150 tungsten 000 buck shot shells at ~2.5 times the muzzle velocity of a shotgun.
[Edit]Well, I tracked down the muzzle velocity and payload weight of the XM1028. Apparently, it fires a 11kg (24.25lb, 169,950gr) payload at 1,410m/s (4,626fps), generating 8,077,712 ft-lbs at the muzzle.[/Edit]