Early in the war the shortage of modern arms required both armies to scour their armories for any available military arm. I saw quite the variety of arms in the hands of volunteers with many being considerably older than the men they were issued to. The M1816 conversion from flintlock was rather common. It was not a bad weapon but many had seen hard service before they were ever converted from flintlock to percussion. While firing buck and ball they were a valid weapon at ranges of under one hundred yards and as lethal as they ever had been. But like their counterparts from the Continent of similar vintage all predated the American manufacturing revolution that led to machine made small arms of the American System. They were hand made instead though I never found the quality to be wanting.
In the 1850’s the US Army took their more modern flintlock arms and converted them to percussion by simply removing the flintlock hardware and replacing the hammer then plugging the flint touch hole and drilling and tapping the barrel for a percussion cone. It was simple and most importantly cheaply done. There were some who worried that the cone in barrel conversion weakened the barrel and made it prone to burst. Though I know of no incident where such happened. Other methods of altering arms from flintlock to percussion were attempted but the US Army chose to use the cone in barrel conversion due to cost.
Muskets such as those above were turned in at the first opportunity for rifled arms of the Springfield pattern or for arms from the Continent such as the P53 Enfield of M1854 Lorenz. By late 1863 most M1816 muskets were back in the armories or in the hands of rear echelon troops such as those assigned to guard prisoners of war or in some cases in the hands of those facing the Dakota during the Dakota War. I know of no incident where soldiers given the opportunity to exchange their M1816 percussion conversions for more modern arms. Though to be brutally honest at close range with buck and ball they were far more destructive than rifled arms. But at longer range men with muskets would be cut to pieces by those with rifled arms.
After the war many were quickly cut down to shotguns and sold for a pittance. I prefer them in their original configuration. That was how they were designed and that is how they should remain.