https://allthatsinteresting.com/heart-attack-gun

Mixed shellfish toxin with water and froze the mixture into a small pellet or dart. The finished projectile would be fired from a modified Colt M1911 pistol equipped with an electrical firing mechanism. It had an effective range of 100 meters and was virtually noiseless when fired.

When fired into a target, the frozen dart would immediately melt and release its poisonous payload into the victim’s bloodstream. Shellfish toxins, which are known to completely shut down the cardiovascular system in concentrated doses, would spread to the victim’s heart, mimicking a heart attack and causing death within minutes.

All that would be left behind was a tiny red dot where the dart entered the body, undetectable to those who didn’t know to look for it. As the target lay dying, the assassin could escape without notice.