This is something I have been wondering: if an assassin wanted to kill their target with a bullet from a sniper rifle, can they dip it in liquid mercury so even if the bullet itself doesn't kill them, the mercury poisoning will? I am looking for scientific basis on how it could be feasible, if it is at all.

And in case you're wondering: No, I'm not planning to assassinate somebody.

No.

If we just dip the bullet in mercury, only trace amounts of mercury will remain. That definitely won't be enough mercury for our purposes.

If we treat a lead (but not iron/steel) bullet for a long time in mercury it will form an amalgam alloy. This can give us sufficient amount of mercury, but adversely affect bullet ballistics. Additionally, an amalgam alloy bullet will likely be removed by subsequent surgery, again, negating our efforts.

If we instead enclose mercury in a hollow bullet, that might work - but it would still be messy and much less effective than cyanide, for example.

Mercury has, in the real world, been used to increase the terminal effect of hollow point bullets (especially a the relatively low velocity of pistol bullets). In essence, a short time before firing (so as not to have all the mercury amalgamated with the lead, but some liquid still left at firing) a conventional hollow point is filled with mercury. It was common to press a small caliber gas check (a soft metal disk of precise diameter, with a raised edge) into the hollow point to keep the mercury in place.

On impact, the liquid in the hollow point magnified the hydraulic effect, leading to much stronger and faster expansion of the bullet. The mercury itself was chemically insignificant; it was the expansion effect that was sought. Worth noting here that my source (an article in a gun/reloading publication, but I don't recall which, much less what 1970s vintage issue) also found that the effect was essentially the same using water or oil; any difference between mercury and water being attributable to the greater increase in bullet weight.

Most who use guns for defense now consider penetration more important than expansion, so mercury tip bullets (which have long been legally prohibited as "toxic", however erroneously) are no longer widely considered an effective modification.

No.

Not even in the form of dimethylmercury.

This extremely nasty organic compound was being carefully used by professional toxic metal researcher Karen Wetterhahn when a few small drops of it touched the outside of her latex gloves. At the time it was not known that this noxious substance could travel through latex. Nothing could be done, she died less than a year later.

To make enough stick to a (hot - >100 celsius) bullet in flight, because it would otherwise evaporate before reaching your target, dipping the bullet would not work, you'd need some water soluble delivery system which releases after impact which would not satisfy the requirements of the question - but careful how your protagonist uses it, lest they succumb the same way.

Edited due to being wrong all over.

Lead is already poisonous, the addition of mercury might be unnecessary if looking to increase the poisoning potential of a bullet. Instead, I would consider using toxin from poison dart frogs in the bullets.

This toxin is highly lethal to most creatures, including humans, and was used by aboriginal tribes in the rain forest. They coated darts with this poison (likely where the frogs got their name from) and used a blowgun to inject a person with it.

You could employ a similar mechanism with your bullets, either coating them in poison or putting the poison into a specialised bullet meant for storing the liquid and releasing it upon impact. If you really wanted to use mercury though, you could fill the bullet with that instead.

Mercury when ingested targets blood, kidneys, central nervous system, liver and the brain. Large doses of Mercury will lead to easily identifiable health symptoms Administration of a chelating agent can help reduce the effects on the body.

Acute Mercury poisoning will lead to death, but generally that death will be due to an already existing kidney or liver damage. There is always the possibility that exposure will cause a failure of the central nervous system, which could lead to death.

In essence, that big new hole in the body from he gunshot wound will be far more fatal either from tissue damage or a secondary infection. But adding the Mercury sure will make your victim uncomfortable until the body sheds the excess amount down to acceptable levels.

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