The Model 1842 Musket was a percussion lock musket, bored in .69 calibre, and built by the armories of Springfield and Harpers Ferry from 1844. The Model 1842 is notable for being the first US musket to have fully interchangeable parts, as well as being the last musket produced with a smoothbore barrel.
The Model 1842 Musket was a continuation of the Model 1795 line of Muskets, which had been developed and re-released in 1812, 1816 and 1822. Hence the Model 1842 used the same basic design of those previous models (which can be traced back to the 1717 Charleville Musket of French origin) although several major differences are found, one being that the Model 1842 utilised a thicker barrel, as the designers predicted, correctly, that the Model 1842 would be converted to fire the Minie Ball (a developed version of the Musket ball which required a rifled barrel). The length and bore of .69in (17.5mm) would remain the same as previous Models.
Crucially, however, the most notable feature of the Model 1842 is the percussion lock mechanism, replacing the flintlock mechanism of the older models. The percussion lock has the advantage of using a percussion cap to fire the musket ball, with the gunpowder is not exposed to the elements due to the flintlock (which strikes the frizzen on a flintlock musket to create sparks which ignite the gunpowder while the powder is exposed) being replaced by a hammer, which strikes a firing pin, which in turn strikes the percussion cap. This removed the issue of misfires occuring in wet conditions.
Another major difference between the Model 1842 and its predecessors, is the machining process that was used, allowing for all parts on the Model 1842 to be interchanged. This also sped up the construction of the Model 1842 meaning that in the 13 years of production around 275,000 Model 1842s were produced.
Smaller companies and armories would be contracted to produce the Model 1842. The majority of these companies would use brass instead of iron for cosmetic features (such as the trigger guard).
The Model 1842 (like its ancestors) used the standard .69in (17.5mm) musket ball (as had been used since the Charleville Musket in 1717). It would, however, be the last .69 caliber musket to be produced in the US, due to the release of the Minie ball ammunition which could cause as much damage as a .69 caliber musket ball at a smaller, .58 caliber, size.
The rate of fire for the Model 1842 was dictated by the user, however the average infantryman was expected to fire 2 to 3 shots a minute. The use of percussion caps slowed the loading process.
The Model 1842 Musket was immediately adopted by the US military in 1844. It was, however, quickly converted to fire the Minie ball ammunition when it was released in 1847, due to its improved accuracy. The barrel was rifled to accomadate the Minie ball. This conversion also saw sights added to the Model 1842, as the original model had not used sights (seen as unnecessary on a weapon that was accurate to a couple of hundered yards at the most).
The Model 1842 would be used in the Mexican-American War and the American Civil War, alongside the other Model 1795 developments. Those engagements, combined with the introduction of the Minie Ball, prompted all major US firearm firms to move away from smoothbore muskets and into rifled muskets of smaller caliber.
The Model 1842, therefore, is a significant step in musket design, being the first musket in the US to be fitted with a percussion lock as standard, the first to use fully interchangeable parts and the last to use a smoothbore .69 caliber barrel. Its replacement, the Model 1855 Musket, was the first American built musket to be rifled and to fire the Minie Ball without modification.