If You mean a vent in breech plug with a short part of about half the bore dia. in face of the plug,this breech is also kown as English style breech. The purpose was to give more consistent ignition from shot to shot, also a more uniform burning process. Well worthy design if using small "primer" charge of fine powder under main FG-FFG load in target shootig-but this is probably more our european cotinental practice than yours. First aplied on pistols, as if I know, because of beig small(er), thus more deviation-sensitive.
Only a vent made in breechplug mass, ending in axis of barell is a predecessor, also with a goal to obtain more consistent ignition. You may sometimes find with FFG and coarser that you do not have any powder just under the nipple with a drum-style breech, as it has to pass a right angle. These types of breechplugs almost eliminated this issue.
A vent from side, just in front of face of the breech plug is a logical evolution from flintlock liner-the same (in that time only) proven configuration, everybody was used to. Easy way for coverting flint- to capslock. Possibility of fastening the ignition was discovered later.
That´s a general view, here from Europe I have no idea about colonial gunsmiths, so that´s someone else´s turn.
