Voldemort met outdoors in a graveyard in the series, once, and that was at a point where nobody but his servants ha any idea he was alive. Secrecy was the Death Eaters' main weapon.
As for assassinating someone like Dumbledore, you could probably do it if he weren't already suspicious enough to take precautions against it, but you could do that with magic as well. Warfare technology would certainly have uses in the wizarding world, particularly for a smart individual, but it's not like any particular combination of technologies and spells is simply uncounterable, it just makes things much more complicated and forces everyone to become more paranoid.
Aside from raising standing shields, taking undisclosed routes or teleporting to safe destinations and all the other precautions one might take, magic might take bullet tracing to entirely new levels. "Find the gun that fired this bullet" and "Find the person who fired this gun" spells may very well exist, or be easy to invent once they're needed.
I'm thinking McGonnagal could set up a decent nuclear defense system too. Charms that detect incoming airborne objects and transmogrify then into pigs seem right up her alley.
In general it seems that magic gives far more defensive options than technological weaponry. These days our defensive options are pretty much MORE ATTACK! But magic has invisibility, shields, teleport, (extra) secrecy and flipping time turners!
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: