Could a...
Ok, here goes...
1) Could a bus hit a Bradley with enough force to knock it off it's tread?
I suppose so, if it had enough speed and hit it broadside. But my money is on the Bradley.
2) Are there any rules in section D regarding ramming?
No, but there was a module put out by the now defunct ON ALL FRONTS that covered ramming in a mini Kursk campaign. I've got it somewhere in that mess I call my basement...sigh.....
3) Did the M1A2 fire in Defensive fire or Prep fire?
The answer to that is YES
4) And what would be left of the bus?
Well, as you know, a hit vs. a soft skin vehicle is not necessarily a kill ie the round could shatter one side and pass out the other unexploded.
HOWEVER, if it did explode--lets assume he used a HEAT round not APDS (armor piercing discarding sabot--or just plain SABOT round), I'm sure that the bus was nothing but shards and pieces of rubber/flesh when it was over.
Can you imagine the fire commands in the Abrams?
TC: Gunner, HEAT, Bus ramming Bradley
GUNNER: IDENTIFY
LOADER: UP
TC: FIRE
GUNNER: ON THE WAY....
Oh, I hope the Greyhound people forgive for what I've done....
Mark