Charge, you ever try to pound a nail through the bottom of a peanut butter jar? One of the plastic ones?
Just a few smacks with a hammer and it's red hot from the energy. The bullet itself would heat up like a mofo on the way through the outer casing. It would be red hot and literally set fire to the filler. I'm not talking full explosion, but burning explosives STILL exert a lot of force. Enough to rip a plane's wing off (rip it off, not blow it off).
Also consider that the standard 5.56mm round from an M4 rifle can punch through a 1/4 plate of STEEL so easily you can see spiral patterns out the back where it exits and where the metal literally folds back over itself. 5.56 is about a .20 caliber round (rounded off), so a 30cal would be a stronger round. Forget about 50cal. 50cal can go through half an inch of solid steel and keep going out the other side. Forget about 20mms. They've been known to fly through the tail of an airplane, through the pilot armor, through the armored seat, through the PILOT, and out the front of the plane somewhere (there were reports of these over the Channel).
I don't think there's any problem with bullets penetrating. As long as the angle is right. Like a shell off the side of a tank, if the angle is flat it'll go in.
I don't think there's any problem with igniting the explosives inside the bomb shell. It's very possible, and is in fact one way of ord disposal today. NOT to be confused with the primer blowing the bomb up, this is like a lower level of explosion, and the shell does not fragment, but it rips around hard enough to seriously damage any structure it's bolted to. [EDIT: The one I saw a film of wasn't bolted but a 100+ pound arty shell jumped several feet in the air and bounced around like a fire hose gone wild very briefly. I'd imagine anything it was bolted to would be stressed to the point of breaking]
I think it would be an interesting feature, someday. I don't think HTC will get to it soon, though.