Re: Steering a bullet.
Well, the Excalibur system solves the fin deployment problem by being fired on a spin-countering discarding sabot to both allow for the projectile's deployable fins and to dramatically reduce the spin with which the actual projectile itself is imparted. The one thing everyone's fogotten here is that the last thing you want on a dirigible projectile is spin. Quite apart from the difficulties in operating any kind of steering system, the gyroscopic effects are a PITA. I found it quite amusing when it was pointed out that the ideal weapon for firing Excalibur and its ilk would be a smoothbore cannon, but they went with the sabot as the US don't have any smoothbored heavy artillery pieces in service right now.
I'd have thought that producing a sabot-discarding round in a similar vein that can be fired from a .50 cal rifle would be difficult, but not beyond the realms of possibility. However, far simpler would be to produce a .50 cal (or similar) dedicated smoothbored weapon to fire such an object from. The bore could then be whatever shape in cross-section was most convenient to accomodate a projectile with deployable fins. Yay! Muskets are back.