One could get around that by having a gun type fission bomb with something other than explosives to launch the bullet (say compressed gas).
You're making this too complicated. As evidenced by the levitate-slowly-to-the-ground spell, they've already got magics in-universe that impede the maximum kinetic energy of an object.
Just surround the entire area with a field that inhibits maximum relative velocities to something an arrow could achieve. No more guns, no more bombs, no more nukes. Problem solved.
I assume that at a certain power level, even magic can't protect you. Atlantis at full power probably couldn't defend itself against, say, a gamma ray burst, a black hole pulling the solar system into it's gravity well, our sun going supernova, or heck, the sun just expanding due to old age.
A spell to protect against incoming shockwaves would probably require vastly more energy than a spell that targeted and halted igniting charges. Although ironically it seems much more muggle thinking to halt a theat with through intricate understanding of the mechanisms than to just pump more power into it.
Update: Discussion has moved on to a new thread.
The hiatus is over with today's publication of chapter 73, and the previous thread is approaching the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: