cool! getting a working turrent was going to be my next step.
The bombs work by tracking collisions, if the the force of the collision is over a certain threshold, it decreases a 'strength' value in a class assigned to m_userData in the shape, As the strength value decreases, the color changes, when if finally reaches 0, I add a large amount of angular velocity to the body the shape belongs too, and destroy it the body in the next time step. I'd be happy to share the code with you, just drop me an email or hop on the box2d IRC channel. email@example.com
It's not a true bomb. For instance, nearby objects will often remain completely unaffected. My idea for a better bomb would be to define a circle that represents the bomb's shockwave radius. For every shape in the circle, apply a force outward from the circle, with the amount of force dropping off the further the shape is from the blast center. I'm not sure if this kind of area based collision testing is supported by the engine though.