In Fallout 3 my main Scavenger weapons were Trench Knife, Chinese Pistol, and the Railway Rifle. I liked how I actually had to conserve ammo for the Rifle instead of being Trigger happy, which was why I used the Chinese Pistol more often. Are their any guns with rare ammo besides the Alien? I'm on Xbox so mods are out of the question.

The Anti-material rifle, but thats overkill,
or maybe a .44 Magnum.

The Brush Gun and Hunting Revolver use .45-70 gov't rounds. I think only two vendors sell the ammo The Gun Runner and Quartermaster Brandon.

Energy weapons on hardcoe mode. It's not that the ammo is particularly rare (though it is to some extent), but rather that they simply weigh too much, so leaving the house with 3000 MFCs is simply out of the question; you can only carry ~600, tops, and several of the best EW consume 2+ MFCs per shot.

Aside from that, probably .44 magnum. .45-75 Gov't is common at some point when enemies start wielding brush guns, but .44 magnum isn't wielded by a whole lot of enemies, making it rare and viable, thanks to the Trail Carbine being a decent gun.

There is a hardcoe more where ammo has weight so you have to watch that, along with hydration, food and rest if you want a more realistic expirience. Other than that most all guns, especially at higher levels will have a pretty good amount of ammo, especially if you buy or reload ammo.

also how are you not "trigger happy" with a 9mm chinese pistol? that thing packed about as much punch as a water weenie. seems like you'd need at least 50+ rounds to kill a single super mutant master on normal.