For centuries, men in combat stood or knelt. It was the only way they could reload, from longbows to muskets. Crawling and prone firing were not part of soldiering in those times. Stealth in combat only came later.
Crawling and prone shooting became more common in the Civil War, as men sought to use cover. Percussion cap rifles also made it easier, but the best were cartridge-firing rifles like the Sharps carbine.
The problem with the crawly guy and prone shooter was when playing with toy soldiers meant actual projectiles. Anything from the tin "shell" from a spring-loaded toy cannon to a hastily-tossed dirt bomb could topple standing and kneeling troops. The effect on the prone and crawling figures was nil. That meant they were excluded from the game or stood up by sticking their feet far enough into the dirt.
You can see the actual firing methods taught to armies here: http://www.thortrains.com/getright/shootr1.html