In federal law and most state law, a percussion or flintlock handgun is considered an antique and can be purchased with no requirement for licensing, NICS check, or waiting period.
BUT, and it is a big BUT, under the criminal code of every state, such a gun is still a deadly weapon and it is legally the same as a Glock in laws regarding concealed carry, assault with a deadly weapon, or armed robbery.
So forget the idea that because it is a "non-gun" for purchase, it is OK to carry it without a license (where one is required). And if you (heaven forbid), knock off your obnoxious boss, it is still murder, even if done with a matchlock pistol.
Jim