I don't know how much "wisdom" I can impart, but here is a brief history. At the end of the Civil War, the Army had millions of rifle muskets, of many descriptions and from many sources. They first determined to sell off the clunkers (odd calibers, foreign manufacture, etc.). They knew very well that the day of muzzle loading military rifles was gone and that metallic cartridge rifles were the wave of the present. But after any war, there is a period of budget cutting for the military ("the war is over, what do we need an army for?) so it would be nice if all those old muskets could be converted to use the new ammunition, saving many dollars (or pounds, or francs, or marks).
So the Army tried several conversion systems and settled on one invented by a Springfield Armory employee, Erskine S. Allin. It employed a forward hinged breechblock, commonly called the "trapdoor". The first model, the Model 1865, was a true conversion; the whole rifle was the same except for the breechblock and the hammer, plus minor changes to the lockplate. The caliber was .58 rimfire so the barrel was not altered. Some 5000 Model 1865 rifles were produced by Springfield Armory.
The next model, the 1866 still used mostly Civil War guns but the caliber was changed to .50-70 center fire, both by lining .58 barrels and later by making new barrels. 52,300 of the Model 1866 were made, all conversions. There was a cadet rifle version, very rare, with only 424 turned out.
The Model 1868 still used musket parts, but the receivers were new, as were most barrels; the caliber was still .50-70. 52,145 were produced.
The Model 1869 carbine and 1870 rifle are uncommon today, only 11,533 rifles and 3422 carbines having been made.
Finally, in 1873, the first truly new rifle was made, even though some CW parts were still employed. The locks, receivers and barrels were all new and so was the caliber, .45-70. Rifle, carbine and cadet versions were made and served as the first line U.S. shoulder arms for 20 years; some remained in service with NG and reserve units into the 20th century.
Jim