The vile (or pleasant) act of defecating into your mate's mouth and proceeding to force the excrement down his/her throat with your penis. Your mate's mouth acts as the muzzle, the exrement acts as the bullet/gunpowder, and your penis acts as the ramrod. Hopefully your mate will projectile vomit immediately after being muzzle-loaded, so as to replicate the firing of the loaded firearm. If you wish to take it a step further, you can hire a Civil War Commander reenactor to shout out commands during the process such as "Prime!" "Draw Ramrods!" "Ram down the cartridge!" and "Fire!"
Dan: Bro, I hired a Civil War Commander reenactor to shout out the proper commands as I performed the muzzle-loader on your mother last Thursday.
Mike: Did she projectile vomit?
Dan: Hell yeah!