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:
You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).
If there is evidence for X in MOR and/or canon then it's fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that "Eliezer said X is true" unless you use rot13.
Perhaps there is a charm in MoR. Although in cannon there were arrows killing wizards.
I'm not really concerned about muggle vs wizard, but rather wizard vs wizard. As was mentioned earlier in the story, any spell you can throw out requires them to expend effort to negate. And guns can spit out quit a few 'spells' per second as well as from beyond unaided sight range. Even if a shield existed, guns would still be changing things by forcing enemies to keep up that shield at all times.
Plus there's also the question of IEDs. They pump out enough damage that I doubt any wizard could withstand one. The IRA was active during the time period, and made use of carbombs; it would be unusual if someone like Seamus Finnigan didn't know about them.
There might be a spell for inactivating guns and/or destroying them.
It's odd that wizards are immune to flame (couldn't be burnt as witches) and yet they're so vulnerable to impact.
It would be interesting to throw a Spell of Coherent World-Building.