Back in the mid-1860s most of the world’s militaries began to clamber to adopt breech-loading systems following the success of the Prussian Dreyse Needle Rifle during the Prussian-Danish War. The Dreyse breechloader had given the Prussians a tactical advantage over the Danes who were still using muzzle-loading percussion rifle muskets.
In 1865, the British Army began looking for a new breechloader. They trialled dozens of designs including one submitted by Johann von der Poppenburg. Poppenburg’s gun used a sliding breech action actuated by lifting up a lever on the top of the action. Here’s a gif showing how it worked (full video here):
Poppenburg’s gun was beaten by Friedrich von Martini’s action and Alexander Henry’s barrel, which when combined became the famous Martini-Henry, adopted in March 1871. If you’d like to know more about the breechloading trials, Poppenburg and his rifle, I’ve written more about it here.