Even before the war, the percussion cap had replaced the older flintlock and matchlock systems for firing a musket/rifle. The copper cap had the advantage of being useful in all weather.
Their manufacture in Knoxville was due to the town’s having one of the few copper rolling mills in the Confederacy. It was one of the manufactures that Union Gen. Burnside ordered destroyed as soon as his troops took the city in 1863.
The copper percussion caps had their disadvantages. Their small size made them tricky to manipulate in the haste and confusion of battle. Hence development of such as the Maynard Tape Primer, very similar to the rolls of paper caps a few boys still use these days in toy pistols, when their parents can find any to buy. But the Maynard system, like the toy caps, was susceptible to moisture.
Both sorts of caps came to be widely known, even to civilians, by such colloquial phrases “at the pop of a cap” and “bustin’ caps.”