https://www.rockislandauction.com/detail/73/109/us-navy-np-ameswm-jenks-mule-ear-carbine

The Jenks carbine was a remarkably svelte and elegant breechloading system patented by South Carolinian William Jenks in 1838. It was tested by the US Navy in 1841, and found to be quite successful. The Navy would proceed to adopt it, and order 1,000 rifles and 5,250 carbines from N.P. Ames in the early and mid 1840s. The last 1,000 carbines were a separate contract which included the use of the Maynard tape primer system, and this contract was purchased from Ames by the Remington company, which manufactured those carbines. The Army also tested the Jenks system, but found it completely unsuccessful - perhaps due to a misunderstanding of the appropriate powder charge and projectile.

Mechanically, the Jenks uses a bolt which slides forward and rear connected to a larger action lever on the top of the receiver. Opening the lever retracts the bolt, opening a round port through which a ball and powder charge may be dropped into the breech. Shortly before the Civil War, most of the guns in Navy inventory were modified to extend this round loading port into an elongated oval, to allow the use of paper or linen cartridges instead of loose powder.

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