Unfortunately, that seems to pretty much undercut the founding father's major justification for 2A, which was that in order to have a ready fighting force to defend the country, the people should own arms and by implication be practiced with them so that fielding an army would take less time and training (“A well-regulated militia”, with “well-regulated” keeping the 18th century concept of something mechanical in proper working order or calibrated correctly).
The 2d Amendment has nothing to do with the defense of the nation. It has everything to do with making certain that the federal government did not have a monopoly on the tools of coercive force.
Many have commented that military strategy is less concerned with the combat effectiveness of the individual combatant than larger deployment of force.
True then just as it's true now. At the time the 2d Amendment was written marksmanship was mostly irrelevant to military tactics. Soldiers lined up in formation and fired volleys of musket fire at soldiers who were also lined up in formation. Yes, there were sharpshooters but they were used in a combat support role. Contrary to popular belief, the revolutionary war was not won by farmers and merchants who turned out with their Pennsylvania and Kentucky long rifles and picked off the redcoats from long distances. The revolution was won by the Continental Army (with a lot of help from France) using the same tactics other armies were using at the time. The only militarily relevant small arms task was how to load the weapon. Military drill at the time consisted of the motions one had to do to efficiently load the musket and how to march and change the formation from columns to lines and back again. These tactics and the necessary skills to support them didn't change until after the Civil War.
Many have indicated that individual training of “marksman” or “expert” is not absolutely necessary except for Marines or Special Forces.
Marksmanship is only irrelevant in the Air Force and Navy where most of the jobs do not require the use of small arms. Everyone in the Army and Marine Corps must qualify as Marksman in order to graduate from basic training or in the case of the USMC, boot camp. Granted that the rating of Marksman is easy to earn, but it is still a quantitative requirement.
Marksmanship training has made huge advances since the NRA was formed in the late 1800s for the purpose of promoting rifle shooting. At that time, marksmanship training was very rudimentary and the military didn't spend a lot of time and money on it.