Burnside carbine action close-up.
U.S. National Park Service Official Web-site, Certified Historic Weapons Program
<a rel="nofollow" class="external text" href="https://www.nps.gov/stri/upload/19th-Century-Carbine-Manual.pdf">Manual of Instruction for the Safe Use of Reproduction Breech-Loading Carbine and Rifle in Interpretive Demonstrations</a>, Page 3