If a magic 50/50 gun randomly kills either the target or the triggerman, then one way to make it a 100/0 gun is to make the target the same as the triggerman. This immediately suggests two applications:
Suicide. I recall reading a sci-fi story a long time ago in which, IIRC, guns were outlawed(?) and so you had to go to extra lengths to commit suicide; the protagonist acquired a bootleg single-use gun, little more than a tube with a trigger attached, referred to ironically as a "Wednesday Night Special." Your magic 50/50 guns could fill the same niche — if your world was dystopian enough, yet safe enough, to maintain a market for them.
Booby-traps. Suppose you place one of these guns with a tripwire so that the person who unintentionally fires the gun stands a 50% chance of death, and the target stands a 50% chance of death, and coincidentally these are the same person.
Now some epicycles suggested by the booby-trap idea: Clearly there's some sort of magical determination of who the "firer" of the gun is. Otherwise, I could just aim the gun at you, push the trigger with a long stick, and with 50% probability I'd have to go find a new stick but at least I'd be unharmed.
Suppose I train a pigeon to peck the trigger. Is that good enough to trick the magic? or does the magic affect the person who trained the pigeon? (Or the person who bought the pigeon from the pigeon-trainer?)
You have the opportunity to create a whole system of rules and regulations around these dynamics. (See What exactly is “Psik Reisha”?) The difference between your system and halacha is that in your system it would be relatively very obvious when somebody got the details wrong!
My brother wanted to kill a deer with a gun, so he placed the gun on a tripod on the ground, climbed an iron ladder into a camouflaged hide, and used a 10-foot length of silver chain to depress the trigger. However, he still died. What did he do wrong?
Dear Confused: We need more information. How many links were in the chain? Was it raining at the time?
Now, "wanting to kill a deer with a gun" might seem like a terrible reason for anyone to court death in this manner; but:
Remember that people are stupid.
The military-industrial complex will very quickly figure out the halachically safe way to use gunpowder weapons, as long as these weapons actually help to kill people. (And they will help, because once you know the halachically safe way to use them, you can use a machine gun just as safely as a pistol. Of course the technique will start out as an extremely valuable military secret, and may stay essentially that way for a long time; see also, atomic weapons.) The technique will eventually become public, at which point private citizens will also be able to use the technique — or as close to the proper technique as they can manage given their limited resources. Compare plinking.
Now, my whole answer is essentially based on the idea that your magic operates by halacha-style rules, which is maybe not what you intended. If you mean that your magic is just inherently unpredictable and uncontrollable, then I think the fundamental answer is that nobody will try to use it unless it offers astronomical rewards (e.g. summoning Mephistopheles, granting wishes, etc). I don't think "maybe killing a dude at a distance" qualifies as an astronomical reward.
If the good effect can be scaled up without a proportional scale-up in the bad effect, then perhaps you would still see them used as weapons of mass destruction. (This is essentially the "suicide bomber" idea with a patriotic spin.) Suppose the bomb dropped on Hiroshima had a 50% chance of destroying the city and a 50% chance of killing the pilot — they'd probably still do it. But suppose it had a 50% chance of destroying the city, a 30% chance of killing the pilot, a 10% chance of killing the general who ordered the drop, and a 10% chance of destroying Boise, Idaho — well, they'd probably call off the drop.