Not to mention the analogy is flawed... The rifle of the 19th century was a specialist's weapon, not by any means standard issue. The M16 is today's "musket", and today's "rifle" is a purpose-designed sniper rifle with telescopic optics. I do not deny that the Baker rifle or something similar may have had shot-for-shot "better" kill statistics than say the M16, but you also have to realize that
1. They had to make the shots count. Modern warfare with self-loading firearms means volume of fire wins the day. A rifleman in the 19th century could not simply spray lead at anything that moved, but you can bet your ass he would if he had the technology.
2. They were shooting man-sized targets standing in the open in dense formations, unable to effectively return fire, at relatively close ranges by modern standards.
Modern warfare is about firepower. Marksmanship is great, don't get me wrong, but the reason the assault rifle is the weapon of the day, and not the bolt-action or semi-auto battle rifle of yesterday, is because it can simply sling more lead. Most ammunition today is going to be expended in suppressing fire and things like that... To say riflemen of the 19th century had better shot-for-shot kill stats because they were better-trained than today's soldiers is a baseless statement.