“...The smooth bore 69 cal. muzzle loading musket remained the standard infantry weapon in the U.S. Army until 1855. There some militia units armed with the “Mississippi” rifle during the Mexican war...”
Not the case.
The United States was one of the first nations in the world to equip troops with a standard-issue military rifle: the M1803. See the final page of the December 2017 print issue of American Rifleman magazine (the “I have this old gun ...” regular feature). It was of half-stock configuration, made at Harpers Ferry. The column author, Garry James, says it was 60 caliber, but Flayderman lists it as 54 caliber.
From 1803 until 1855, US land forces used two standard long arms, issuing rifles to some units and smoothbore muskets to most other units. It was recognized that the rifle offered greater range and accuracy, but was more difficult and slower to load; the musket, on the other hand, offered quicker loading, and thus superior short-range firepower (shots per minute). Rifle troops were allowed a full minute to load; musket troops were required to load four rounds per minute.
Later rifles were the M1814, M1817, and M1841 muzzle loaders. All were 54 caliber. The first US-issue breechloaders were made on the Hall pattern at Harpers Ferry, starting with the M1819 in 52 caliber.
(The M1841 earned its nickname “Mississippi Rifle” thanks to its use in the Mexican War by a Mississippi regiment under the command of Jefferson Davis, a graduate of West Point and a Regular Army officer at the time. Davis later became President of the Confederacy.)
Numerous experiments were made to combine the quick-loading capabilities of a smoothbore with the accuracy and range of a rifle: the Baker, Thouvenin, and Delvigne systems were some. These were all English or European in origin.
The problem received its best solution in the 1840s when Capt Claude-Etienne Minie of the French Army developed a hollow-base bullet. A small iron plug was placed in the base; when the arm fired, the hot gases rammed the plug into the soft lead of the projectile, expanding it to grip the rifling.
Capt James Burton, a US Army officer assigned to Harpers Ferry, perfected Minie’s concept by reshaping the hollow base; the plug was dispensed with. The US Rifle Musket M1855 adopted in that year was of 58 caliber, firing the new projectile. Effective range more than tripled compared to the smoothbore musket with no loss of rapidity in loading, and the dual-caliber system was obsolete.
“Minie balls” were used extensively during the Crimean War and the American Civil War.
Thousands of M1841 rifles were rebored to 58 cal. Many 69 cal muskets were rifled; many flintlocks were converted to percussion as well.
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