Mort Kunstler, whose painting this is, is one of the leading sentimentalists of American Civil War art. He does Union pieces, too, but seems to prefer Rebel ones, probably because they sell better.
Kershaw’s Brigade of South Carolinians held the sunken road on Marye’s Heights at Fredricksburg in December, 1862, stopping multiple Union charges until the battlefield was littered with Union dying and dead.
THE TEXAS SCRIBBLER 
Is that barrel fire depiction realistic? I’ve never seen one of these old guns being shot in reality.
Yes, black-powder weapons do that. Even if they’d had flash suppressors, which they didn’t, I doubt they’d have worked. Cordite to replace black powder wasn’t invented until late in the 19th century.