coil gun ammo holder

I am currently creating a coil gun within a small nerf pistol. Everything is going well, it all fits within the gun quite nicely actually. But one problem I've realized is that there is nothing keeping the ammo within the barrel (a bic pen tube) 

I would like to hear some ideas on how to keep the ammunition (which is currently undecided, if that helps. I'm considering either small steel balls or nails) within the barrel until the coil is activated. It is worth noting that because of the design of this particular gun, the trigger is not a lever, but instead a simple sliding mechanism that hits a button situated right behind it.

One option I'm considering is using an electromagnet hooked up to a NC (normally closed) switch also hooked up to the trigger. The magnet would keep the ammunition in place, and pulling the trigger would turn it off. I have already added a sliding switch to turn on and off the gun, and I could wire the second magnet to that switch as well. The complication I foresee arising from that is that the electromagnet may still have residual magnetism for a few milliseconds, which could hold the ammunition in place for longer than it takes the capacitors to discharge.

thoughts?