Light cavalry wasn't light because that held any major intrisic values, but because mounting all your cavalry on big battlecavalry horses would be extremely expensive, and the big horse anyway didn't give any advantages in the important recon role. But if you pitch light cavalry in battle against heavy, the light cavalry is almost everytime going to loose.
Typically ACW cavalry was used for recon (not good enough for battle cavalry), but against the GA they are up against a huge number of very professional and experienced light cavalrymen. In the "battle of recon" the CSA cavalry is likely to be swept away by superior numbers of GA cavalrymen having better tactics also.
Cavalry relying on their firearms, be it pistols or carbines, is a waste of good horses, and was only feasible when the horses were no good for anything but carrying the trooper to a position where he can dismount and act like infantry (the original role of the Dragoon). In the skirmishing battles CSA cavalry armed with rifles might be able to do some sniping with their rifled carbines, but it wont keep them from being swept back by the GA cavalry. If there is one situation I wouldn't like to be sniper in, it's against good cavalry (and using smoke heavy black powder). You might pick one or two off, but in no time they're behind you and pick you off, even if they have to throw rocks at you.
To stop cavalry with firearms you need accurate massed fire, and that simply wasn't possible from horseback - rifles or not.
The problem of getting close enough would be acute if you charge against units in stable and coherent formations - it already was in the musket age - for infantry as well as cavalry. But in order to have your army deployed in stable and coherent formations, you need good intelligence on when and where it is likely to meet the enemy, and that is where the light cavalry comes in, and the reason why cavalry kept its recon role far beyond the battlefield role.
And I'm not talking about the single battalions or regiments being ordered in square in time, but much more of your armycorps being converged for battle in the right time and place. That art Napoleon was a master of and the allies took long to learn, but anyway you have no chance at all if you're swept aside in the recon battle.
At Gettyburg the CSA fielded some 7-8000 cavalry in one cavalry corps, while the GA at Leipzig 1813, well past the prime of Napoleonic cavalry, fielded some 40.000 men cavarly organised in five cavalrycorps plus typically a cavalry brigade to each armycorps and then of course the famous Imperial Guard of 8000 men cavalry (incl. in the 40.000). I just don't see how the CSA can win the recon battle when up agaisnt a force of such numbers and quality - rifles or not.
Regards
Steffen Redbeard
That is why the battlefield role of cavalry