There's no need to bring whisky into it. Isn't he just saying that the land was so productive that they could get more than a barrel for each shock. A shock is obviously how much corn you can harvest from a certain area of land and barrels are what they would measure the yield by.
I've just never heard of storing or measuring corn in barrels.
My wife has worked with a lot of corn farmers in the midwest and in Appalachia, including in Kentucky. Hang on a sec. I'll go ask her what she knows . . . .
Okay. She says it's measured in bushels and usually stored in silos. (I knew that much.) She adds that it's sometimes stored in barrels, although it's prone to going bad that way. She confirms (I think we all know this) that Appalachian subsistence farmers sometimes make whiskey themselves, so that might be what it's about.
But I did find the text, and you're right, Piscean, the little bit I scanned didn't mention making whiskey. (I don't think I want to read the whole thing. That's Untaught's job.)
So I'm ready to agree that they're storing the corn, not mashing it.