I know several folks who like to argue the advantage of the smoothbore musket over rifles. However, I have never found an instance of regiments discarding their rifles for smoothbores. That settles the question for me; after all, who knows better than C.W. veterans?
Lots and lots of people. Certainly there are several instances of regiments
passing up the chance to re-equip with rifles.
If lots of people in the early war started with smoothbores (which is true, we know this) and some fraction of them declined to re-equip with rifles (also true - Irish Brigade, Texas Brigade) then the people in those regiments would be actively preferring the smoothbore over the rifle.
Now, as to the actual properties of the weapon. Here's the thing - at the usual firefight range of the Civil War (which was about 100 yards averaged over the whole war) the smoothbore is actually not a very inferior weapon. A
perfectly aimed M1842 musket will hit a torso (2 foot target) 100% of the time at 150 yards, and what that means is that in most Civil War firefights the limiting factor on the performance of the weapon is the performance of the man, not the performance of the long arm - if you aim perfectly you will hit whether you have a rifle or a smoothbore. At that range the heavier bullet of the smoothbore does more damage and is thus a superior weapon, while buck-and-ball gives chances for light wounds where a rifle would have scored no wound (and is thus also superior).
At longer range, the rifles become superior - and the Enfield becomes superior to the Springfield.
The next question is - how could a veteran judge the quality of his weapon? How can he tell if he needs the greater range accuracy of the rifle over the smoothbore?
The answer is that he has no way to tell. With typical ACW hit rates (about 0.5% per round) it's quite possible for a Union soldier to have fought at Antietam, Fredericksburg, Chancellorsville, Gettysburg and the Wilderness without ever having hit a man - and because he's firing as part of volley fire, he has no way to tell that, or indeed if he's hit more than a dozen people.
The only way to determine how good you are with a long ranged weapon is to do practice with it, which is to say to train with it. But the Union pretty much never practiced firing, and indeed in November 1864 Warren reported that his command (5th Corps) contained nearly four thousand men who had never so much as fired a musket:
'The command... consisted, first, of the First Division... 4,707 strong, of which 1,247 were ignorant of the manual, and 2,803 had never fired off a musket. Second, of the Second Division... 4,704 strong, of which 104 were ignorant of the manual, and 812 had never fired off a musket. Third, of two brigades of the Third Division... of which 298 were ignorant of the manual and 298 had never fired off a musket.'
The only way these men have to judge whether a weapon is good or bad is how modern or "cool" it is, and a weapon from 1863 is clearly more modern and "cool" than a weapon from 1842. But that's all they have to go on.