Well if a laser is a true cylinder then if u were to only see a portion of the cyclinder flashing past ur eyes all u would see is a rectangle?? So u can use billboards to represent a laser but u don't rotate ur billboard so it always faces the viewers camera.
So if u consider a laser with 2 points, point A where is originates from (probably a gun), point B it's destination or current position. Then u draw a rectangle with a height equal to the diameter of the laser with one end at point A and the other end at point B. U can rotate the rectangle by the vector AB so that the viewer can see the maximum surface area of the rectangle.
Should the laser fire directly at the viewer then draw a circle using a billboard to create the effect.
If u use billboard u can create the 3D effects if u have really cool textures.
opps.... I said the hieght of rectangle should be the radius of the laser beam, in fact it should be the diameter.