Regardless of medium, whenever there's a futuristic, science-fiction war going on, there are lasers. Laser guns, laser swords, laser cannons — laser everything. Now, this isn't to say that lasers are an impossibility in the real world. In fact, the U.S. military has kept an eye on developing high-powered, laser-based weaponry since the 1960s. Even today, the U.S. military is using lasers to heat up objects, like missiles, to take them down with speed, accuracy, and ease.
But here's why the sci-fi staple, as we know it, would suck in the real world.
6. The shot itself
The problem with lasers as seen in popular films like Star Wars is that they don't obey the laws of physics. A laser gun used in combat would feel more like the pen you use to play with cats than any kind of real rifle. Applying actual science to the pop culture weaponry shines a light on how terrible they'd actually be.
There are many works of fiction that employ laser weaponry, so it's hard to pinpoint all of the problems. If you want to be precise, just know that if the blast moves at a rate slower than 299,792,458 meters per second, then it's not a laser. Since you can actually see them move in films, they're plasma — so we're going to assume this discussion is actually about plasma weaponry from here on out.
5. The cost to produce the weapon
This may not be too much of an issue given that futuristic civilizations often have an entire planet's or galaxy's GDP at their disposal, but it's still worth mentioning. The parts needed aren't the problem — it's the power supply that creates the laser and directs it into a single blast.
The power supply would need to be powerful enough to create a blast that deals significant damage. So, you're looking for elements higher on the periodic table. Even if a fictional, galactic empire had the money, based purely on how unstable radioactive elements above uranium are, you can assume that the means of mining or synthetically creating the power supply needed would be insanely expensive.
4. The weight of the power supply
Unless the power supply is explicitly described as some impossible, fictional element, it's safe to use uranium as a scientific starting point for theorizing because it's naturally occurring, stable enough to last more than a few seconds, and, presumably, findable anywhere in the known universe.
A peanut-sized lump of uranium can produce roughly the same amount of energy as 600 pounds of coal. That same peanut-sized lump would approximately be 10cm cubed. That lump alone would weigh 20 kilograms (or around 44 lbs).
Sound heavy? That's only the beginning. Shielding the wielder from radioactive exposure so that they don't immediately get cancer would also be a serious concern. Coincidentally, one of the few effective shields against uranium is depleted uranium — which weighs nearly just as much.
3. The heat after each shot
Now that we've explained the fuels and costs involved, let's break down what a plasma blaster is actually doing. Plasma is considered the fourth state of matter; a substance that is superheated past the point of being a solid, liquid, or gas. If all the kinks were worked out and a power supply could heat up whatever projectile is being fired, it would also need a barrel and firing chamber durable enough to withstand the heat.
A good candidate for the round being fired is cesium because it has the lowest ionization energy and turns to plasma somewhere between 1100 and 1900 degrees Kelvin. The most common element with a higher melting point that would be suitable for weapons manufacturing is boron. Using these elements could ensure the weapon doesn't liquefy upon pulling the trigger, but the person actually firing the weapon would be undoubtedly toasted.
2. The speed of the shot
"Laser" weapons used in most sci-fi films are slow, roughly 78 mph according to Wired. Keep in mind, the muzzle velocity of an M4 carbine is 2970 feet per second — or 2025 mph. Projecting a round by igniting gunpowder simply wouldn't work with plasma weaponry. Logically speaking, the best way to quickly send plasma down range would be with something like a magnetic rail gun.
The high-energy output needed to superheat cesium would also need to electromagnetize the boron barrel to fire the round. That being said, heat has a demagnetizing effect on all metals. So, even if some futuristic civilization figured out how to heat a cesium round to near 1100 degrees Kelvin without losing magnetism, it'd be damn hard to get the round going 78 mph. In reality, given the length of a typical rifle's barrel, by the time the round emerged, it'd move at roughly the same speed of a slow-pitched baseball.
1. Sustained fire
Now let's summarize all of this into what it'd mean for a futuristic door-kicker.
The weapon would be far too front-heavy to accurately raise into a firing position. The uranium-powered battery would need to be swapped out on a very regular basis (which are also heavy). The time it would take to superheat a cesium round to the point of becoming plasma would be far too long. The slow-moving round fired out of implausible railgun would be far too inaccurate to be used reliably.
All of this brings us to our final point: the second shot. On the bright side, there would be little backward recoil, much like with conventional firearms. The second round would also require much less charging time. But the heat generated from the first round would brittle the barrel and make holding the weapon impossible any — let alone fire like a machine gun.