Theoretically, plasma rifles could exist. The basic principle of plasma weapons revolves around the fact that plasma has some magnetic properties, and can be directed with either an electromagnetic pulse (EMP) or a rail accelerator, also known as an EMP beam.
There is almost no difference between the two methods of directing plasma, except that an EMP by itself spreads out in a ring around the transmitter. This method is only practical for plasma bombs, which the United States Navy is rumored to be developing at the China Lake Test Facility, though without the use of EMP. The rail accelerator, on the other hand, directs the EMP, hence the secondary name of EMP beam. It is the fundamental principle behind the rail gun: a metal rail or tube is charged and discharged at a very high rate, normally in excess of 30 Mhz, with a very high electric potential difference, upwards of 10,000 volts, which produces an electromagnetic field running the length of the rail or tube. The material used for the rail or tube (how about I just call it a barrel?) is what is known as a dielectric, a material that can be rapidly charged and discharged without retaining a static charge. (This rapid discharging creates small EMPs.) The effect of the dielectric is such that it creates an anti-gravity field in the direction its discharging surface is facing. In the case of a rail gun, the discharging surface is facing the center and slightly towards the end of the barrel, creating a frictionless barrel for the ammunition to pass through. (Note: since the discharging surface creates small EMPs, the EMPs are directed and focused into a beam due to the inward facing discharging surface of the dielectric. Hence the name EMP beam.) After the barrel is charged, the ammunition is inserted into the firing chamber (an extenxion of the barrel, also dielectric to produce the anti-gravity field) and charged with another high voltage, opposite the barrel of the gun, but not enough to keep it from being attracted to another high voltage component that will be at the end of the barrel. This muzzle piece has a steady charge, opposite the charge of the ammunition, so that the two will be attracted. They both retain the same voltage, effectively doubling the difference in charge. Once the ammunition is charged, it is released into the barrel, with the anti-gravity field pulling it along at a very high speed, possibly up to the speed of electricity, which is 604,500,000 feet per second in a vacuum. As the ammunition reaches the charged muzzle piece, the muzzle discharges, sending the projectile out of the barrel and along its path. The amunition for a rail gun would most likely be a very dense metal with massively strong magnetic properties, perhaps depleted Francium.
A plasma rifle can operate on the same principle as a rail gun, just replacing the ammunition with plasma. Plasma is defined as a substance which is undergoing constant nuclear reaction. The electrons produced by such reactions creates strong magnetic properties within the plasma, which can be used by an electric field to attract the plasma to it. In essence, a plasma has the same magnetic properties as metals do. Therefore, the plasma rifle operates the same as the rail gun, except the muzzle velocity of the plasma rifle is greatly reduced due to the fact that plasma is much more dense than the sabot rounds used by the rail gun. A very small amount of plasma is required to do massive amounts of damage,however, so a person may not need to carry that much plasma with them to have enough to destroy, say, a whole infantry battalion. The reason behind this is that plasma cannot be contained in anything less than a magnetic field, as it is extremely hot and can melt even carbon, which has the highest melting point of any of the known elements.
This post is getting a little long-winded, so I think I'll end it here. And yes, I was bored when I wrote this; somebody in Disorder's "Plasma-rifle" post asked if a multi-ammo gun could exist, and I started to explain the plasma rifle's feasibility instead. Oh well . . .
