Here's a semi-thorough description of Vash's guns. Take it or leave it . Be warned, this section contains MANY spoilers, so watch where you step. In fact, don't read more than a little bit of the first gun description if you don't wanna be spoiled.
Six-shooter - Uh-huh. This is the biggie, the gun Vash uses most often. Across the barrel it says "45 LONG COLT *insert tiny pic of wings that says "AGL" between the two of them* FACTORY AGL", which means that it fires 45 Long bullets, a specific type of bullet, and Colt is the type of gun. The "Angel Arm Factory" is a cute extra, I guess indicating where it was made (*spoiler* although it was made by Knives *end spoiler*). Above the trigger it says "RF Auto Ejecting Double Action", which means the fired bullets eject automatically and double action means it's semi-automatic. Thanks to Kevin for the info ^_^ . It has a twin gun (or brother gun if you'd like to be ironic) that belongs to Millons Knives. Actually, this gun was MADE by Knives (it took him a year while Vash waited patiently outside). Knives gave it to Vash as a gift, but Vash shot Knives after he found out Knives wanted him to kill people with it. In a panic, he stole the black gun too and ran. Knives took off one of Vash's arms to get the black gun back. This gun appears for the first time in episode one, isn't fired until episode 5, and I believe it appears in almost, if not all, episodes in the series. Shoots rubber bullets in addition to the regular kind ^_^; From what I hear, it's construction is upside-down, but I could never figure out why. Apparently not a very good gun that can be counted on for everyday use, it was 3 inches off from only 10 yars. It's also a very heavy gun (why is it that all these guns are heavy except for Meryl's derringers?). The weight could be attributed to several other elements not in other guns, mainly the mechanisms necessary for this gun to fuse with his right arm and become the Angel Arm. See, this IS an important gun! It kinda crunches and expands and there's something glowy inside when it fuses with his arm... but more about the Angel Arm in it's own paragraph. Just thought it was an important enough part of the gun to mention up here ^_^ Was once repaired by the legendary gunsmith Frank Marlon.
Machine Gun - Does this have a fancy name? Vash's left arm is missing, so therefore he has a prosthetic one that transforms into a wicked machine gun. Used for the first time in episode 9 when he vaporizes the spider-androids, but not really revealed until episode 12, when his regular gun ran out of bullets and he needed something to threaten Monev the Gale with. Also rips it out in episode 14 to destroy the Demon's Eye on Dominique the Cyclops (yea, he shot a woman in the eye with it o_O;), and used again in episode 16 to try and counter Rai-Dei the Blade, but it wasn't fired. Also used in episode 21 when he fires on Leonov's puppets and on Hopperd the Gauntlet, episode 24 for various purposes, and episode 26, where Knives disarms him of it in their battle. To operate it, the black glove he wears around his arm shreds, his hand moves off his wrist and goes down to the trigger, and his wrist turns into the gun barrel. It is a suprisingly big gun, actually ^_^ In episode 20, he gets an upgrade though, so that instead of his hand folding down, the gun pops out of the top of his arm. The upgrade's also a hellofa lot more powerful, since when he initially fires it the kickback sends him across the walkway-thingie. The prosthetic hand on this one is apparently a lot more smooth as well ^_^; When not deployed, it just appears as a grey, metal arm, nothing remarkable about it at all. Think of this as the "oh shit" gun, since he only uses this when he's desperate or lacking bullets or the actual silver gun.
Angel Arm - The big mother of all guns. He had two of these, on both his left and right arm, but he lost his left arm, so now there is only one; the one in his right arm. It's sorta literally built into him, he was born with it, and it seems to be a biological-type gun. Has to do with his non-human heritage, as Knives is the only other person born with them (Legato has an Angel Arm, but it doesn't count since it was originally Vash's arm o_O;). While not deployed, it's a normal enough arm, nothing odd about it at all. But when the gun is activated, it turns into a massive nuclear device, capable of destruction of cities. He used this gun to destroy July way back when, and he used it again to destroy August and blow a crater in the Moon in episode 16. Both times it was against his will, the first Knives did it to show Vash what he was capable of , and the August incident Legato was just screwing with him since Rai-Dei was about to kill him. Even when pointed to the sky, it has the ability to vaporize the city, but oddly not to take life (I'm sure it could if it was pointed directly at someone [anyone but Knives, it didn't kill Knives when it was aimed at him in July], but Rai-Dei was still standing when the rest of August was levelled). The way this works is that Vash must hold the silver gun in his right hand, then a compartment opens up revealing a glowy ball. The arm and gun kinda fuse together, and the ball of light ends up in a kind of caged area in the middle of the gun. The whole thing emits a really bright light, and has mass destruction capabilities. Legato claims that Vash is the only one that can do this, but I know Legato has one and Knives has two, so it's possible that the Angel arms fuse with other objects... but I dunno. I think in the manga Knives can fuse with knives... but I'm not even really sure. That's just an educated guess. Anyway... mass weapon of neuclear destruction, yadda yadda... It appears in episode 16 to destroy August, 18 in a flashback, 24 it's activated but Vash manages to stifle it, and 26 Vash voluntairly activates it and it appears in all it's glory, angels, feathers, and all. As an interesting side note, Vash did not know he had an Angel Arm until Knives activated it for him and caused him to level the city of July.