That don't matter so much...you must block the original fire attack and hook attack,then make a custom ent with model/sprite, add a beam or w/e...set it velocity in direction of player aiming , then hook touch with that ent, and make damage to player that is touching it. Also you can emit a custom sound, and replace the model of original weapon, add some recoil,or screen fade effects and so...
A good example can be ->
http://forums.alliedmods.net/showthread.php?p=175993 'plasma shot'