Magic Gun Fail

My Gun is Not Magic
Swords and sorcery gaming is rife with magic weapons, epic items, and exemplar equipment, but no such equivalent is present in modern and futuristic games. There are a variety of reasons that this is not so, and also a variety of ways to bypass this limitation.

Artisan vs. Mass Production
Martial weapons, such as swords and axes, were made one at a time by a smith with a hammer, an anvil, fire, and determination of flesh over metal. The process of becoming a smith was based on apprenticeship and the keeping of secrets of the art. An apprentice would create his best work to his ability and present it as a final test to rise from apprentice to master smith. Guns were treated much the same way until the 18th or so century, each piece being unique, as well as dangerous, temperamental and difficult to use.

Guns are mass produced, and rather than a smith and apprentice it is an industrial press stamping out pieces, motorized lathes turning barrels, and the like. Men made swords, machines make guns. The cost of a gun is not comparable to the cost of a sword, in reference to time periods. A knight would spend more to purchase a basic sword than a soldier would to purchase a basic rifle. Exotic swords would become even more expensive as they would be a challenge to make each time. Meanwhile the gun, once the machines are set up, can spew out an endless stream of rifles and pistols.

Magic vs. Handwavium
Magic can go a long way before violating the suspension of disbelief. We accept flaming swords, dragons with too small wings, and people conjuring lightning, poison clouds and other oddities out of thin air with nary a blink of an eye. This doesn't apply nearly as well to modern and futuristic weapons. While many magic-y types of weapons can be created with basic sci-fi and some modern weapons, they do not have the same feel or impact, mostly because almost anyone can get them. A flaming sword is unique, a clip of incendiary rounds somewhat less so.

Flaming Weapons
Tracer rounds, phosphorus rounds, incendiaries

Lightning Weapons
Particle cannon, plasma cannon

Dancing Sword
Autogun turret

So while the basic abilities can be replicated, it is a function of technology, which is widely available, rather than a unique and extraordinary weapon. Most Handwavium, such as hyperdrive, plasma cannons, and the like are accepted, just as magic is accepted. It is when it is applied to creating uber weapons that it is rejected, electro-nanoprobe blasters, biomutagenic hyper rays, and such that are packed with technobabble and handwavium are panned in favor of magic swords with a mish mash of backstory and motivation.

Symbolism
A sword has two major symbolic aspects in comparison with guns. A sword is a symbol of chivalry, an age long since past, or skill martial and honor on a field of combat, yadda yadda yadda. Guns rather lack the mystique of swords as you don't hear of someone going on a sword rampage, a child stabbing themselves with a sword, or other things. While a knight carries a sword, it is a mercenary, or a terrorist who carries a gun. This is also highly apparent in Star Wars, as the Jedi eschew firearms in favor of, yes, a laser sword. The other image, which the gun also has but not to the same extent is that of a phallic symbol.

How to Circumvent the Mundane Gun
Guns can be made magic, but then that takes away something fro the sci-fi and modern setting. In this case, a magic gun is a cop-out and therefor unacceptable.

Master Gunsmith
In this day and likely in the future, there will still be people who learn to make guns by hand. These weapons will be unique and likely have much higher quality and tolerances than mas produced guns. This would be bonuses to range, accuracy, and damage, but unlikely to produce special effects.

Alien Technology
When the rest of the team has machine guns, the guy with the organic pulse rifle is much like the warrior with the flaming sword. Alien technology uses liberal amounts of handwavium, but produces the desired effect, a special weapon can be used, and being alien tech it is theoretically unable to be reproduced by human's current means.

Exotic Ammunition
A machine gun will fire whatever you put in it, be it a steel jacketed round, an incendiary round, or an exotic iridium isotope round that causes a millinuke explosion. Exotic ammo would be able to reproduce odd effects beyond the regular.

Highly Experimental
Take the current setting, and then borrow a gun from 20 years in the future, and give a PC the prototype. While a new machine prototype is unlikely, this is more fun with energy and exotic weaponry. Being experimental, it can fail or potentially blow, and it is limited in that it will be some years if not decades before the weapon reaches mass production.

Lost Technology
Like the alien tech, this avenue exploits the backwards looking fall of super high technology and survivors stealing what they can from supply dumps. More prevalent in post apocalyptic settings, can work in regular sci-fi settings that have a long back history, like Warhammer 40K or Battletech/Mechwarrior.

Sentient Weaponry and Vehicles
K.I.T.T is one of the best known talking vehicles, and the future would certainly have vehicles and weapons with onboard brains. Smart tanks, smart guns, smart chainsaws, the possibilities are limited only by imagination.

Body of History
Some swords were only famous for the warriors who carried them, and the same can be said for iconic guns and their owners, such as Patton and his pearl handled pistols, or Buffalo Bill and his rifle. These weapons are likely to simply be of superior quality and good repair.

Summary
Magic guns fail because they rarely meet the requirements of a magical weapon. A magical weapon must be special, in that it is highly limited in availability, and thus rare. It must have abilities that set it above, or apart from contemporary weapons of it's class.