External missles really don't make sense. What happens when a 50 kiloton missile gets hit by another 50 kiloton missile?
{...snip...} the missile must be armed and set to detonate. Just damaging a nuke isn't going to set it off.
well, there could be a 1:9999999999999999999999999999999999 freakass occurance where the original explosion is just right to set it off. 
It's pretty long odds, and unlikely to occur the first time out. That's not to say it couldn't, just that it would take a long time, but of course by that time, you don't really have to worry about it, because by the time such an event actually occurs, you've already died of old age, boredom, or the freaking nuclear missiles that have been going off next to your hull...
Also, sorry for being a jerk back on page one. The turrets on page 2 show a great deal of promise.