"The Model 1842 was the last U.S. smoothbore musket. Many features that had been retrofitted into the Model 1840 were standard on the Model 1842. The Model 1842 was the first U.S. musket to be produced with a percussion lock. Like all Model 1816 derivatives, the Model 1842 has a .69 caliber barrel that was 42 inches in length. The Model 1842 had an overall length of 58 inches and a weight of 10 lbs.Like other smoothbore muskets, many Model 1842 muskets had their barrels rifled later so that they could fire the newly developed Minie Ball. " Wikipedia -This info sounds correct to me- I would suggest the "improved" musket had been rifled, possibly in 1850? - Barrett