Ooooh... I have a way to do this!!!!
1. Make a custom CMP, consisting of a very long set of squares with alpha-channel, glowing textures on them (so that the edges are properly blurry and it looks like a rounded cylinder). Make it... oh... I dunno... maybe 2000 units long.
2. Make the weapon fire said CMP as its projectile. Make the projectile move very, very slowly. Give it a refire rate that's slower than the time the projectile stays alive.
3. Use FLModel Tool to resize a copy of crate_blue.sur (it's a box) so that it's the same size as the "beam"... so that now the "beam" will have a valid SUR (for determining hits).
And... voila!!!!! You'd have a beam that looked suitably large and impressive, functioned as a valid game object... caused no unusual lag... and would disappear as soon as it struck a valid game object!!!