Place a path_track where you want the bomb to start, and another at the bottom so that the bomb just touches the floor. change that func_physbox into a func_tracktrain and set the speed to something close to what you want (maybe start high, like 400. I don't know, I got that number from 32 ft/s/s which is accelaration due to gravity, and converted it to inches/s/s, but play around with that number). Parent the bomb to this brush entity. Also at the bottom, place an info_particle_system and put in the name of one of the particle systems used in the payload maps (they should be in the VDC, but you can also just open up one of the maps in hammer and have a look. They alll start with sdk_). Then to get it going, set the button to enable the prop and start the func_tracktrain forward. The go the path_track at the bottom and add an output disabling the model again (OnPassed...), and another to trigger the explosion particles.
