I don't think Nuclear Weapons explode just by shooting them, they're not bombs after all but a chemical reaction.
http://images.wikia.com/fallout/images/8/87/Fo3_Liberty_Prime.png a single metal beam and exposed cables. http://www.youtube.com/watch?v=ly8_wwGuU2Q&feature=related, one Vertibird just flies over him shooting at nothing and the other stops and circles in front of him to shoot his torso, a clear shot from behind with missiles at the exposed cables would have done it; not even that infact, the Enclave shell the bridge, why didn't they just shell the Pentagon, that crane goes and they win, why was that crane even there anyway? Maybe I'm just a Enclave [censored] but as far as I'm concerned it's crystal clear that the Enclave should have won in seconds flat and the whole main story is a joke, a cruel, sick joke which has reduced my favourite faction into everyone's target dummies, "New Vegas enemies are boring, not Fallout without Enclave to shoot! and Super Mutants."
I know you weren't looking to pick an arguement but I wanted to moan at something so more, and don't call me a crybaby Personally I think I'd sooner be dead than red.