BB Gun
The first weapon you get in the game, it&'s given to you during the tutorial section. It&'s not much use in a firefight, but some players hang on to it for sentimental reasons.
Pistols
The 10mm pistol is another weapon you get early on. It&'s semi-automatic with a 12-round clip. It&'s useful early in the game and against weaker enemies later on. Most players keep one handy, because ammunition for it is more common than for most other weapons. There is a silenced version as well, but it&'s considerably rarer. Later in the game, you may find a .44 Magnum revolver. It does a lot of damage, but ammo is rare, so you&'ll have to use it sparingly.
A .32 pistol and Chinese pistol are also in the game, but neither of them is as effective as the 10mm.
Submachine Gun
The 10mm submachine gun is a great replacement for the 10mm pistol. It&'s fully automatic, so you can eat through its 30 round clip pretty quickly. Don&'t expect to be making too many trick shots with it as its accuracy isn&'t great.
Assault Rifles
The standard assault rifle is fairly common throughout the game. Its rate of fire isn&'t as high as the submachine gun, but it does more damage per shot. Many types of enemies carry this weapon, so ammo shouldn&'t be a problem. The Chinese assault rifle is slightly more powerful, and it is harder to find. They both use 5.56mm ammunition
Shotguns
The double-barrelled shotgun is the most common shotgun in the game. You only get two shots before you have to reload, and its not very accurate, so use it at short range, then switch to another weapon instead of reloading. The combat shotgun has an eight round clip and can be fired semiautomatically. It&'s an extremely powerful close-quarters weapon.
Rifles
The Hunting rifle will be your go-to long range weapon for most of the game. It uses the moderately common .32 caliber ammunition and has a five-bullet clip. It&'s accurate enough for you to line up headshots before your enemies even know you&'re coming. Later, you&'ll find the Sniper rifle. It has a scope, for accurate targeting at even longer range. The .308 ammunition it uses is quite rare, though.
Big Guns
The Flamer, missile launcher, minigun and Fat Man fill out this category. They&'re all quite heavy, so don&'t carry more than one or two at once. The Flamer, a short range flame thrower, is one of the most deadly close-quarters weapon in the game. The minigun has the highest rate of fire in the game, going through its 240 bullet drum in seconds. It&'s best at short to medium range. The missile launcher is great at medium to long range, capable of taking out groups of enemies with one shot. The Fat Man is the undisputed king of dealing widespread damage, firing tactical nukes at your target. Be sure you&'re at some distance before letting one of these off.
Energy Weapons
There are two main types of energy weapon in Fallout 3: laser, which use the Energy cell ammo, and plasma, which use Microfusion cells. The both come in pistol and rifle varieties. Plasma weapons do quite a bit of damage per shot, but their projectiles travel slowly, making them most useful at medium range. Laser weapons fire beams that strike their targets instantly. They fire faster as well. Late in the game, you&'ll starting encountering enemies with a Gatling laser gun, one of the most powerful weapons in the game.