There's a very large difference between 'backfire' and 'wounds will appear on the user's body instead'. Can you break down the percentages of each?

@Halfthawed Sadly numbers are not my strong point. I mean to imply that there is an equal chance for all the above listed outcomes to occur.

pretty sure thats how early european firearms is, which is prone to backfire, i think the percentage is even higher than 50%, yet it end up get developed through stubborn trial and error anyway.

A weapon which is lethal to the owner 50% of the time would never be used. For one thing, to become competent with a firearm, you need to fire several hundred rounds (all but guaranteed death) and firearms would never be developed as all their inventors die during development. Suicide vests for your cultists are more realistic and safer.

You will need only one gun for a duel.

@Alexander Russian roulette. WIthout the roulette part.

"Gun" might be a poor way of describing this gunpowder-based contraption, because people are familiar with guns acting differently. It sounds more like magic missile wand that's shaped like a rifle.

@Dragongeek - Not only that: The weapons would be prohibitively expensive to supply if they kept destroying themselves.

With a 50% chance of backfiring, it makes more sense as a poor trap of some kind than as a weapon.

How exactly do you get to a 50% chance of "lethally backfiring"? How does your magic manage to read the shooter's intentions to such a degree that the only possible results seem to be the desired outcome, or the exact opposite? Because without that mind-reading aspect, you're left with a 50% chance of something completely random happening, of which there's probably only a slight sliver of a chance of that leading to the shooter's death.

If people use grenades, do they also get the wounds even if they aren't "shooters"? If people use cannons and the "firing mechanism" is something like a weight falling after a candle burns a rope, they are also "shooters"?

Tarantino would still use those in mexican standoff

@Falc As I said, it results in chaotic, unpredictable effects. Hence it is a soft magic system.

If you want your world's chemistry and biology to resemble ours, making common substances magically unstable is going to have serious side-effects. Sulfur, nitrate and potassium are essential for life; charcoal will appear wherever there are forest fires. More generally, life by its very nature relies heavily on controlled chemical reactions that release energy, such as respiration. These same reactions, if uncontrolled, can release energy fast (burning wood) or very fast (exploding sawdust or flour).

Changing this in any meaningful way is going to involve some serious changes to physics, not just a magical plaster covering the chemistry of gunpowder. As jdunlop's answer points out, even if you somehow manage to prevent all fast releases of energy from chemical reactions, there are other ways to store and suddenly release energy.

One possible reading of the question is that this magic only kicks in when guns are used in war. Is that the case? If so, guns could be perfectly safe for hunting, pest control, target shooting, etc, and certainly would be useful enough for the technology to be developed. Just a thought.

@Dragongeek Such a weapon would definitely be used to avoid capture. Between 50% chances of hitting the shooter or the target, it basically becomes a 100% reliable device for that.

@creative-username Yeah, but if you have a 50% chance of a chaotic result, then it has to be truly chaotic. The actual odds of that backfire affecting the shooter would be (at most) 1 in (your entire human population). The odds that it will be negative let alone lethal to the shooter are maybe a tenth of that? And that would be about as likely as the backfire causing the sun to pop out of existence. If anything can happen when that gun is fired, then ANYTHING must be possible.

Your question presents a common issue with magic in fictional settings. Magic, like everything else, must have its own laws, behaving in a consistent manner that is ultimately discoverable so that reliable predictions can be made. If the only "law" for your magic is that it does whatever you want it to do at any given moment, then your story is one deus ex machina after another.

Do the characters in this world use magic itself as a weapon? Either way, I don't think anyone would want to use a gun if this were the case. An interesting idea I had though, maybe you could develop a really lucky character who is really powerful because he has been using a gun and has not yet been injured/killed by it...

@RobinSaunders Have you read much low fantasy? "magical plaster" is very useful in those stories. I think you and Falc are looking at it expecting too much precision.

If you used a sabot mechanism, then the gunpowder is pushing a wooden plug, not the bullet. Getting hit with a wooden plug is annoying, but the bullet is lethal. Is this cheating?The bullet would then still hit the enemy. And if a user wears bullet-proof armor (a bullet-proof was a mark on armor to show it couldn't be penetrated) does the lethality rate go down? All sorts of things could reduce the effects of the magic if the negative effects are variable. 15% lethal wouldn't be quite so bad...