A muzzle loader AR upper receiver can include a muzzle loader AR pattern upper receiver configured to be coupled with an AR pattern lower receiver. The muzzle loader AR pattern upper receiver includes a body with a barrel opening. A muzzle loader barrel is coupled to the barrel opening of the muzzle loader AR pattern upper receiver. A muzzle loader AR rifle can include the muzzle loader AR pattern upper receiver and the AR pattern lower receiver coupled to the muzzle loader AR pattern upper receiver. A method of forming a muzzle loader rifle can include: providing a muzzle loader AR upper receiver; and coupling the muzzle loader AR upper receiver to an AR pattern lower receiver. A muzzle loader kit can include the muzzle loader AR pattern upper receiver and any other component, such as a ramrod, barrel, or AR pattern lower receiver.