Bukkit

The idea taken from this: http://gfycat.com/DismalFortunateKestrel (vanilla client)

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Pros: Cool graphics with new aspects for Crackshot, Particles, Block break effects, explosions and a lot more modules can be associated with current modules. Completely new purpose for the projectile type. Most likely it will not be broken at every patch.

Cons: This effect may be broken if Mojang is willing to change it's AI behaviour //again//. Checking where the laser is hitting may be unstable. Particle effects on the beam is not safe to be implemented since it always mess up at every major patches.

How to implement: Either holding invisible guardian on your hands for easy method. Right click to summon invisible guardian shooting the laser beam and right click to cancel the effect or something. Or, simply hook into the guardian behaviour for harder but stable effect. It is better and most likely not be broken at every patch.

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Sample of working laser guns in vanilla can be found here: http://www.dragnoz.com/hand-held-lazer-youtube/
On this video and code, he is summoning an invisible guardian and making him shot the lasers.