Cordite

A smokeless firearm propellant used by the British during the first world war. It was replaced by US powders before world war II and later in the century by German manufactured powders. In fact, the last of the british gunpowder factories shut down at the end of the 20th century.

Although cordite is long gone in our universe, it lives on in the writing cliches of the 41st millennium. It's almost impossible to read a description of a gunfight in any W40k novel without the smell of cordite being mentioned. Bolters? Cordite. Autoguns? Cordite. Lasguns? Charge packs full of cordite.

Priests of Mars seems to be the first W40K novel to use the term "propellant" in place of "cordite."