Concept:
A 'Heavy' Elemental gun, similar to the Magnus and the Iron Slug. However while the Magnus/Ironslug specializes in killing single targets the Beam Cannon specializes in taking out groups.
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Appearance:
Essentially a Mecha Turret with a handle, but I drew up some basic sketches anyway: http://img543.imageshack.us/img543/9418/mtgpng.png
[sorry about the low resolution, ImageShack seems to hate my guts. Active and Inactive are when the gun is firing and not firing respectively.]
(Alternatively the gun could be mounted ON the arm of the wielder, similar to the Mega Buster from Megaman.)
EDIT: And now I've realized the Energy Symbol on the 'Inactive' gun points the wrong way. Sorry!
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Attacks:
Normal Attack: Fires a powerful beam that can go through a single enemy, the beam does 50% less damage once it has gone through a target. This gun has a very small clip (2 shots, perhaps even just one shot.)
Charged Attack: Fires a larger beam, this doesn't do that much more damage than the normal attack but can go through multiple targets, though it still loses a bit of damage as it does so.
Alternative Attacks: Fires a powerful beam for about 3-5 seconds, the Knight can turn (but not move) while the beam is being fired. The charged version deals more damage and lasts for 5-7 seconds. Gun would still have a 1-2 shot clip.
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Names & Descriptions:
2-star: Beam Pistol
"A basic energy pistol that uses Mecha Turret technology to fire powerful beams of energy."
3-star: Beam Cannon
"The larger cousin of the Beam Pistol, the Beam Cannon is nearly as powerful as the Turret, despite being much smaller."
4-star: Supercharged Beam Cannon
"A Beam Cannon modified for extra power, the cooling fins of this gun grow so hot from the extra heat that Knights have been known to fry eggs on them."
5-star: ???
"???"
This reminds me of Kentard's Retro Beam idea, but possibly more balanced. Perhaps all of these could be part of a similar alchemy line - Beam Cannons do more damage on the first few targets but lose power with each target hit, Retro Beams do less damage overall but keep the same power through all enemies?