|"Buck" from a "buck and ball", Wikipedia|
If my personal library can't help me, my first stop with Civil War questions is the Missouri in the Civil War Message Board. I asked if these wounds could come from the common use of “buck and ball”, in which three (or sometimes more) pellets of buckshot were wrapped in the cartridge paper along with the ball.
Consider for a moment that your position takes a volley from a hundred of the enemy. One hundred bullets are now singing through the air at you. Not a pleasant thought. Also there are 300 buckshot pellets coming at you, so you are three times as likely to get hit by a bit of buckshot as you are by a ball.
Assuming you only get hit by one pellet, you'll probably only be wounded, and probably only “slightly” wounded. Now add to this that many soldiers, especially Confederates and some Union militia, only had shotguns or squirrel rifles and were firing at an unsuitably long range for those weapons, and you can see why there were so many “slight” wounds.
You can even be slightly wounded by a cannonball. I read of one incident of a shell bursting right next to a soldier. The force threw him into the air and his trajectory was stopped by the trunk of a nearby tree. He was knocked out cold, but when he came to he was unscathed except for some nasty bruises.
Someone pointed out that buck and ball was only used in smoothbores, not the Enfield or Springfield rifled muskets with their deadly Minié balls. Smoothbores were only used early in the war. That had slipped my mind. I’m sure some smoothbores still saw action in later years with the Union militia and Confederate forces. Even as late as Price’s invasion in 1864 there were many unarmed rebels in the ranks. I would think they’d grab anything available. But in essence the poster was right. The answer must lie elsewhere.
Civil War author Bruce Nichols replied, "I read in Connelley's 1910 Quantrill and the Border Wars, pages 318-9 and in Castel's 1962 William Clarke Quantrill: His Life and Times, page 113 that the west-central Missouri guerrillas developed the wartime practice of reducing the amount of gunpowder in their revolver loads both to save precious powder and to reduce pistol recoil to improve accuracy, especially from horseback. I think this was especially helpful with repeated or continued shots. Guerrillas from this region were influential in passing along such techniques and tactics to other Missouri guerrillas they encountered, so this practice may have spread."
I'm thinking this may have been common practice with regulars in the Confederate army too, since they were often short of powder.
Another researcher pointed out that slight wounds may have been caused by "spray" from whatever those bullets hit. If soldiers were hiding behind rocks or fences, and bullets hit those barriers, all sorts of stuff would be flying around. There might also be "shavings", bits of the bullets sheared off while coming out of the barrel, creating an unintended "buck" along with the "ball".
Ask a question on this forum, and you always get a wealth of answers! I'm thinking that all of these explanations contributed to the high number of slight wounds in the Civil War. Not that these wounds always stayed slight. One poster mentioned his great-great uncle received a "slight" wound in the side at Hartville in January 1864. He was listed as dead the next month.