U.S. "Trapdoors"
Just to add a little to what has already been posted about "trapdoors".
The first conversions were the Model 1865 done as described above and used a .58 Rimfire cartridge. Only a few were made as the Civil War (how is a war 'civil'?) was winding down.
Then the Army changed to a .50/70 centerfire cartridge and used a .50 liner for the old .58 barrels. These were often referred to as "Second Allin Conversions" and designated Model 1866. They did see action on the frontier including the "Wagon Box Fight" near Ft. Phil Kearney in 1867. There followed newer models that used new breeches and later new barrels with only a few parts used from old Civil War rifled-muskets.
In 1873, the U.S. Army adopted the .45/70 cartridge and the Model 1873 rifle which was made from all new parts. The 'trapdoor' mechanism stayed in service until the smokeless powder Model 1892 Krag-Jorgensen rifle in .30/40 Krag was adopted. The old 'trapdoors' remained with the militia for several years to the extent they saw service in the Spanish-American War and Phillipine Insurection through the early 1900s. NO models of the 'trapdoor' were ever made or 'modified' to use 'smokeless' powder! These days we do use very mild smokeless loads in our old trapdoors but caution is the rule.
The American "trapdoor" action is hinged at the front and flips forward like a trapdoor, hence the nickname. If you want to see them in action in a movie, watch "Springfield Rifle" with Gary Cooper. It is historically inaccurate but does showcase the trapdoors in action.
One note on the original loading for the .45/70 is that the government cartridges were folded head copper cases which would 'stick' in a hot breech. The small extractor would tear through the soft rim and a case would be stuck in the chamber. For the Infantry, they just pulled the under barrel cleaning rod and knocked it out. The Cavalry had no such implement. Soldiers were commonly advised to carry a cheap knife to help pry the stuck case from the chamber. One account of the Little Bighorn Battle had two of Reno's officers occupied during their seige on the ridge using the wooden 'barrack's rods' to clear carbines with stuck cases.
And a few comments about the Martini-Henry action:
The Peabody and the later Martini-Henry action hinged at the rear and the front of the breechblock was dropped for loading. The block then made a nice loading ramp. It was probably the most efficient single-shot action developed in the 19th Century (my own opinion). It had a large and sturdy extractor unlike its U.S. counterpart. However, the British, like the U.S. had to 'monkey' with the ammunition. The .577/.450 MH round was first loaded in what is best described as a composite case. It had a proper solid head but the body was sheet brass coiled around the head to make the case. After extensive firing without cleaning, the head would sometimes extract without the rest of the brass, rendering the rifle unusable. The British finally went to a drawn brass case to cure their otherwise fine design's ammunition problems. I had a M-H for awhile and a full power load was brutal but could have probably dropped dangerous game with no problem. Some of the stories from the Rorke's Drift battle tell of the soldiers, after firing constantly all day and part of the night, no longer being able to hold the rifle to their shoulders. They had tried switching shoulders during the day but by the end of the battle both were sore. They resorted to bracing the rifle against anything else in firing to avoid the recoil. Only an extremely well designed single-shot rifle could have sustained that rate of fire for such a long period. All the soldier had to do was keep stuffing cartridges into the breech, no delays reloading magazines or clips were required.
I apologize for the length of my post but I love these old rifles. I have an 1873 Springfield and a Martini-Enfield in .303 British. They are fun to shoot and harken back to the "Age of the Rifle" when small lines of men stood their ground against great odds and won the day. No matter the sins of the governments that placed them in that position, the soldier had to survive by dent of a good supply of ammunition, discipline and "pluck" to see the next sunrise.
). 
