It actually looks much like a contemporary iron (dumb) bomb, apart from having somewhat swept-forward wings. Modern real nuclear weapons have a distinctive shape to them (at least American weapons such as the rather archaic B57 or the newer B61).
Here's a good question though, as seen on the Tyllium processing plant raid in the first season, Adama uses conventional weapons in a "tossing" manner to blow the facility-a maneuver which requires some degree of gravity. Would it be possible for large objects, such as asteroids/small moons (something large but not so large that FreeSpace can't simulate it) or even some vessels to have gravitational affects? These affects might not neccessarily affect the spacecraft as its artificial stability system might appear to make that gravity negligable... unless you want a full-blown semi-Newtonian gravity model. Which in most cases is unlikely. But then again, you never can tell...
As far as OTHER weapons go (not to move too far from topic), I've noted that there has been some discussion in the past pertaining to what possible weapons the "Blackbird" could carry without straying too far from canon. As seen in show (I've not seen that episode), the ship does not seem to have a gun/cannon of sorts. Would it be possible for the vessel to carry a single/twin MEC-A6 gunpod in place of a heavy weapon missile?