The US Military Actually Experimented With Ricin-Coated Bullets
They also talked about creating a 'dust cloud' of ricin toxin that could be "inhaled through the lungs."
This would, of course, have been devastating to the enemy, not to mention a painful way to go.
From Global Security:
Two methods of weaponizing ricin were explored: bullets and shrapnel coated with ricin toxins or a 'dust cloud' of toxin toxins inhaled into the lungs. The thermal sensitivity of ricin to heat posed a problem as the firing of bullets and shrapnel would disable the toxin. Military authorities decided to delay development of the dust cloud until an antitoxin was created.
Ouch, they didn't create an antitoxin first? Might want some water around if you're playing with fire, no?
In any case, sixteen major nations signed the Geneva Protocol in 1925, promising never to use gas weapons in war ever again (comically, the U.S. version sat in the Senate until ratification in 1975).
Chemical weapons were eventually banned through and through with the signing of the Chemical Weapons Convention in 1997.
The U.S. didn't develop an anti-toxin until 2009.