Stupid. stupid. stupid.
The civil war was not deadly because of "rifling" per se, which had been used in war for almost a hundred years at that point, but because of the Minie Ball, which made it easier to load a muzzle fire arm, since the ball expanded with the gas pressure behind it to fill the rifling.
Thus riflemen went from being an expensive specialized type of troops of use more in skirmishing and sniping than main line battle, for example two companies were raised under Morgan in the American Revolution who became known as "Morgan's riflemen," famed for their accuracy, to being the default kind of infantry. This increased the length of the killing zone dramatically, it also increased the effectiveness of dug in troops dramatically. Thus in Crimea, when the French were just implementing the Minie ball, the Light Brigade could charge the Russian embankment, with cannon and small arms defending it, and take it, where as such charges, a staple of Napoleonic tactics, where elaborate ritual suicide in the American Civil War.
The Minie Ball, not rifling, is the innovation that dramatically increased the volume of long range fire, and lead to the notable disconnect between tactics and military reality in the American Civil War
But wait, there is more!
LV Anderson further goes on to compare cannister fire from artillery to a machine gun. That's stupid. The key to the lethality of the machine gun is the ability to lay down suppression, and zero in on targets, thus concentrating the fire on the hapless soul that moves in sight, as well as fire quickly on command. A cannister might be compared to a giant shotgun, or modern M18 Claymore Mine.
LV Anderson, you're doing it wrong.