Littledude I was thinking about that for a long time, and then I played a DeathMatch on Dippindots server (Dippin Has epic skillz in which nobody that I know can possibly surpass, or even imagine of having the skill to mid air no scope headshot with a scrappy pistol. No one.) and he had everyone start with a gun called the mp7. I used the left mouse button to shoot, and then after many kills later I learned the right mouse button shoots a small grenade.
This "Plasma Pistol" could do almost exactly that. When you hold down the left mouse button it will continuously shoot small shots with not too much damage per shot. Like maybe 15-25 and I say to 25 because I might make this gun a power weapon but I probably wont. And then to be realistic, when you press the right mouse button, it will shoot a large blast that will inflict exactly 99 damage. Just like Halo, it will not be a one hit kill, but if the enemy is damaged then they will be killed. When you press the right mouse button, it will shoot this large blast, and then it will begin a reload sequence of the green ring blinking red and the gun moving in front of you face and the top of the gun opens to let out the extreme heat of the charge. Just like Halo.
Looking ahead, I wont make something so incredibly unoriginal and (I hope no-one steals...) a incendiary grenade, like a HE grenade but it will burn and slowly kill. :D And then I might go crazy and make a razor blade launcher, Hehe kinda intense but...

Whew long post, but I'm a talkative person so... Yeah XD
NEW SCREENY! :D
In this photo I show the 3D detail of the pistol, even though there is not much. XD (Me, I love Photobucket :3)