Ive used laser sites in some of my images made from primitives/cylinder, you will need to scale it down to about 1.5 or 2 % and then turn up the y_scale to what ever you need (any thing over 100000) pose it to where you want it in the gun, and then parent  it to the gun. Next go to the material room. Set the diffuse_color to what ever you want the laser beam color to be, diffuse-value to 1. specular _value to 0. ambient to a darker colour than the diffuse_color. Ambient_value to .3 . set transparency to .75 and transparency_edge to .25

And that should about do you, have a play around with the settings to find what suits you most.
good luck