Shooting seems to have its share of pee-freaks, spreading unsubstantiated tales. Here's a few I can recall from the internet and pre-internet books and magazines. As far as I know, only one is true:
1. Long ago, to make gunpowder (black powder) stronger, soldiers urinated in the powder and then dried it.
FALSE - Urinate in powder and you'll get soggy, unusable powder. During the long, stinky process of making potassium nitrate (saltpetre) -- one of three ingredients in black powder -- it was learned centuries ago that the urine of heavy drinkers was particularly good for this use.
The microbes that converted the organic material to potassium nitrate thrived on it. This is probably where this falsity stems from.
2. Urine was used to clean black powder guns.
KINDA - It was probably a field expedient to wash away heavy fouling, but not a regular practice.
I recall reading that, during the Korean War and its bitter cold, GIs in foxholes peed on the action of their M1 rifle to keep it from freezing over with ice. I am uncertain this is true, because the M1 rifle is an exceedingly dependable arm.
3. Machine gunners in the trenches of World War I peed into the water jacket of their machine guns, when they ran low on water, to keep their guns firing.
I've heard this many times, and even saw reference to it at the World War I museum at Ypres, Belgium (where three of the war's bloodiest battles were fought). I believe it, but only as a a field expediency.
4. The Viet Cong peed on the sharpened punji stakes they used in booby traps, to cause greater infection.
But then, urinating edged weapons before battle was known long ago to cause infection.
Ancient armies also smeared feces, mashed insects and other nasty stuff on their arrowheads and blades to induce infection.
There are probably other tales of odd uses for urine, aside from being a fertilizer. Few of them are likely true.
"And lo, did I see an ugly cat. Smoke. Brimstone. Holes in parchment. And this ugly cat was much amused." --- The Prophesies of Gatodamus (1503 - 1566)