Is there a two-handed gun outside of Megaton in Fallout 3? Where can I find one?
Outside of Megaton, to the left behind where the saloon would be (facing in towards megaton) there is a 'hollowed out rock'. I don't think it shows on the pip boy, but if you move your cross hairs over it, it will say something like 'Open hollowed out rock'.
There are 2 or three molerats around, at least at lower levels.
Inside is some food, a note and a sniper rifle with some ammo. Can't remember if there was anything else there.
edit: removed ascii map because I can't make it work :P
check the fallout wiki here: http://fallout.wikia.com/wiki/Hollowed-out_rock_(Fallout_3)
I'm pretty sure you can find some decent weapons in Springvale school just northwest of Megaton. There's probably a hunting rifle or two, possibly even an assault rifle. It's chock full of raiders, but that's usually my first stop to pick up weapons, ammo, and a bit of loot. If you're careful you shouldn't have to take on more than one or two raiders at a time.
I'm assuming you really mean "where are the closest two-handed guns to Megaton"; see my comment to the question. If so, you don't even have to leave town. Accept Mr. Burke's offer to detonate the bomb, turn him in to Lucas Simms — don't say you're kidding, you'll lose the option of telling him later — and you'll be able to take the sheriff's Chinese Assault Rifle after the scripted sequence.
If you don't care about negative karma, you could also just kill Lucas Simms or Jericho right off the bat; each of them carries a Chinese Assault Rifle. If you do get to loot Lucas Simms's corpse for whatever reason, you'll get his house key; the game doesn't make it clear, but the same key opens the Megaton armory, which contains lots of weapons including some more two-handed guns. Beware, there's also a Mister Gutsy guard inside, and he'll take low-level characters apart pretty easily.
If you follow the Wasteland Survival Guide quest line, you will find the quest called
Traversing a mine field that will lead you to a minefield where you will encounter an NPC named Arkansas who wields a sniper rifle.
This area is fairly dangerous, so tread carefully. You may find some bugs or molerats to kill but will most definitely encounter frag mines that can be somewhat difficult to disarm in certain spots.
In any case, find and kill Arkansas to acquire his sniper rifle.