What are trapdoor springfields?

Some articles on trapdoor springfields, springfield, trapdoor:

Springfield Model 1870 - Variants
... The trapdoor Springfields had originally been designed as an inexpensive method of converting Springfield Model 1863 muskets into breech loading rifles, which ... The original trapdoor Springfields had replaced only the firing mechanism, and had used a barrel liner to reduce the caliber from.58 to.50 ... This barrel liner proved to be unreliable in the field, and later trapdoor Springfields, like the model 1870, abandoned the barrel liner method and used newly manufactured ...
Springfield Model 1868
... The Springfield Model 1868 was one of several model "trapdoor Springfields", which used the trapdoor breechblock design developed by Erskine S ... Originally, the trapdoor Springfields were created to convert Model 1863 Springfield rifled muskets to breech loading rifles at a relatively low cost ... The Model 1868 was also the first trapdoor conversion to use the cartridge extractor covered by U.S ...