Eternal Novice's PM
There are rumors going around in the Zone about a newbie who couldn't even make it from the Swamps to the Roadside. He was armed only with a PM, unable to afford anything better. Eventually, he talked one of the stalkers into getting him a better weapon, offering his PM and a bit of cash as payment. The stalker found the request amusing and simply took the pistol to a mechanic, who made some modifications to it that proved to be quite successful: an improved bullet feeding mechanism increased the rate of fire and a reshaped magazine increased its capacity. In all other respects, the pistol was the same as it was originally. No one knows what became of the newbie, but this version of the pistol went into mass production, and now anyone can get their hands on one.