This is a sex move when a woman takes off her underwear and puts them in her mouth. The male then rams his penis in her mouth stuffing the underwear down her throat.
When having intercourse in "doggy style" you extract the smokeless tobacco or dip from you mouth and place it into the girls anus. Then using your penis, you ram it into her anal cavity.
Pioneered in the great state of Alabama, the Birmingham muzzle loader occurs while chewing tobacco and having anal sex simultaneously. One removes the ball of tobacco from his mouth, sticks it in the anus of the girl, and rams it in with his penis. The Birmingham muzzle loader ladies and gentleman.
When one partner takes a dip (chew) out of their mouth, put it on their finger and shove it in the others anal cavity. After this takes place fornicating insues.
When you are doing a chick in the ass, right after you pull out her anus is still expanded and you insert your dip into her anus then stick your dick in and continue. Hence you are loading the muzzle.