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Bullets are far more violent then Phasers.

Phasers emit a beam of light ,not unlike a green laser pointer, that appears to make no visible mark or wound on the person and could just as eaisly be putting the person to sleep as it is killing them. Not to mention many of the times you see a Phaser used in Star Trek they have it set to "stun" which temporarly knocks a person out rather then killing them.

Bullets on the other hand make large flashes of light and very loud noises, which then enter the person at a very fast speed usually resulting in severe internal damage and massive bleeding. Not to forget, Guns & Bullets are real.