The Linear Launcher, which appears to be some kind of advanced energy weapon firing a large blue lightning ball like projectile, is very powerful, vaporizing the mutated Alexia Ashford in a single hit. It does however require time to charge for use, and a slight delay to recharge if the shot misses.
The weapon most likely uses Plasma as its main projectile, because of the effects it caused on Alexia.
In Darkside Chronicles the linear launcher is given a little bit more background, in which it is explained that it was designed in secret by Alexander Ashford specifically to kill Alexia should she ever mutate and become a threat.
Resident Evil CODE:Veronica
"Anti—B.O.W. Linear Launcher An advanced weapon developed to handle B.O.W.s."
The Linear Launcher is normally only available after Alexia transforms into her final form in which it will destroy it in one hit. However, Alexia can be very difficult to hit with it because of her quick movements. In Battle Game, it can take out just about any enemy in one hit. An exception being Gulp Worm which can take more than one.
It is also an unlockable weapon in Code: Veronica, it is obtained after beating the Battle Game with A rank with all the characters. It can only be used in Battle Game however.
Resident Evil Survivor 2 Code: Veronica
The Linear Launcher can be used Resident Evil Survivor 2 Code: Veronica at the end of Stage 5 during the secret boss fight with the Nemesis. In Dungeon Mode, it is part of Chris' inventory. To unlock it for either Claire or Steve in Dungeon Mode, you have to score 300,000,000 or more. Source.
Resident Evil: The Darkside Chronicles
"This powerful, long-ranged weapon must be charged after each shot."
It is an unlockable weapon in Resident Evil: The Darkside Chronicles if you complete hard mode. (the Secret Chapters involving Krauser do not have to be completed to unlock it. )This weapon is different in the Darkside Chronicles by having a slower fire speed compared to the original. The Charge speed can be upgraded to make it charge faster however.
Resident Evil: Survivor 2 - CODE; Veronica
If the player obtained all the jewels in the game, after defeating the Tyrant in Stage 5, Nemesis T-Type will appear and the Linear Launcher is the only weapon capable of killing him. Chris also uses this weapon in the Dungeon Mode. It has infinite ammo.