Psychokinetic weapons are among the rarest of weapons in the Cosmic Era. The only things more rare and elusive are one of a kind arcanotech weapons. The concept behind the psychokinetic weaponry is to replace internal components and actions within the weapon with the parapsychic ability of the weapon user. Thus, in the hands of a mundane human, robot, mutant, the weapon is utterly useless. PK rifles and pistols do not have triggers, safeties, sights, scopes, or other accoutrements.
Psychokinetic weaponry was created by accident. Gun adepts were already pushing the limits of how good projectile weapons could be, in terms of machine quality, and the smoothness and responsiveness of said weapons. The pistols and rifles that were made, often by hand, for these bullet ballet dancers make even the best firearms of the Petroleum Era seem like crude slugthrowers.
The first psychokinetic weapon was a rifle made by a gun adept who had become a gunsmith. He worked and made a number of exceptionally high end rifles, and used them for his own work. In a shadowrun on a Hamburg, Germany job, he was injured, and his gun was damaged. Rather than going cudgel with the weapon, or bleeding out, he removed half of the trigger mechanism, lifted the rifle to his shoulder, and fired it through sheer will. The weapon only worked because of the enormous amount of resonance he had with the weapon, but after returning to his forge, he perfected his creation. This was the first true PK rifle, and it had no trigger, no trigger guard, and had the same simplicity and beauty as a katana.
Psychokinetic weapons remove the chaos and variables of the human holding the weapon, and of things that influence the trajectory of the weapon. There is no trigger pull, no twitch of springs, firing pins, no mechanical influence on the weapon. The Gun adept has no need to compensate for anything other than the wind and the density of the air.
In game terms, psychokinetic weapons have a much larger critical range, making their accuracy bordering on supernatural. In addition to having enormous critical ranges (on dice rolls), the damage rolls of psychokinetic weapons are open ended, allowing for a master shooter to inflict terrifying amounts of damage.
One of the reasons that psychokinetic weapons remain elusively rare is that said weapons cannot be mass produced. They have to be hand made by a gunsmith who is themselves psychokinetic. The most effective weapons are used by the gunsmiths who made them, as they have imprinted their very soul into the weapon and it will function as if it were an extension of their body. For other gun adepts, the weapon is still excellent, but doesnt function quite as well. For a normal person, a gun without a trigger is as useful as a sword without a blade.
There are only a handful of gun adepts who are also gunsmiths, and these few individuals are not keen on keeping a Guns 'R Us stocked with their wares. They are reclusive, and when they do make a weapon for another adept, it is in commission. Thus, the number remains small. There is also a small chance that an adept will not be a good fit for the gun they are given, and this can seriously degrade their effectiveness.
Psychokinetic weaponry is a niche market, one that caters to the most elite of marksmen. This very limited pool of users limits the actual value and utility of these weapons, and renders their value on the market far below what one would expect. They are novelties, conversation pieces, and the hidden aces of said bullet artists.
Evidence Article 1 - the most common way to come across a PK weapon is it being recovered from a crime scene, and the gun adept who used it is the target of the shadowrun blood hunt. This can be a dangerous game as the gun adept in question is likely aware of being followed and will defend themselves with all of their firepower and stupendous accuracy.
Gun for Hire - the PCs hire a gun adept to join their shadowrunning team. Formula is simple, pick a quirky character/actor, give them a memorable weapon that has been made into a triggerless piece of art, and watch the carnage. (The first cluster I think of is the cast of the movie Red)
Expendable - the PCs have been hired by the gun adept and their job is to make sure the adept has time to make the finger of god shot they need, be it diversionary raids, flushing out the target, or just holding the line until the adept has their shot.