The M60 variant you saw is a combat engineer vehicle (CEV) with the main gun replaced with a short barrel gun for blasting through fortifications at close range. Utter overkill in that situation.
You are ALMOST right. Everything I've read about WACO says that these were tank recovery vehicles that had booms, not guns, short-barreled or otherwise. They were like, or similar to, the vehicle w/boom that was used to help pull down Saddam's statue in Baghdad.
The booms on the vehicles at WACO were used (possibly modified, but I can't remember) to bash through the wooden structure and pump in whatever gas (CS if I remember right) they were using against the men, women, and CHILDREN inside. This gas was forced in as a mist (which I read, was highly flammable in that form) through the boom of the vehicles.
And thus began the wholesale slaughter of largely innocent civilians by our own government jack-booted thugs, led by Clinton, Reno, and the rest of the criminals in charge at the time.