This might be surprising, but the number one factor in finding shells —at least in this day and age —is endurance. Since many Civil War sites have already been hunted, today's shell hunter has to be willing to recover shells at depth and in areas already littered with iron "trash." They must be willing to dig signals that most other relic hunters either don't want to dig or who can't detect deeper signals with their own machines.
Since shell hunters have to hunt in "all metal mode," one is fortunate if the area they are hunting is "clean." Usually, most shell hunters have to contend with deep iron signals that end up being "trash," but nevertheless must be dug just to rule out. This can become demoralizing. My deepest shell was recovered at nearly 4 feet. But I can assure you, I dug several other holes just as deep only to come away with big chunks of old farm equipment.
My advice to you is to find an older relic hunter who has found a shell somewhere but who is not a huge shell hunter. Many relic hunters just hunt for buckles and bullets, etc, not artillery shells. But some of these guys often luck their way into finding a shell or two. If they feel like they have cleaned the area of buckles, buttons, etc, they might share with you where a shell was found. Better if they can take you there and point. If you can find someone who will tell you that, I would go to that exact spot and dig every single iron signal that lies within a 30 yard radius. That's where the endurance comes in. Good luck!!