Reducing dried chestnuts to size of corn kernel
The issue with swinging a hammer is that you have to aim it at a nut that you're holding in your hand. It's not efficient because it requires concentration, and you're going to be doing it a lot. You will definitely smash your fingers some of the time. Instead of a tool that you aim and swing, you want something that hits the nut every time, ideally with little effort on your part. Here are some suggestions, arranged roughly in order from easiest/cheapest to most difficult/most expensive.

Drop the hammer onto the chestnut instead of swinging it. Hit the nut with the flat top of the hammer. This will work best with a heavy hammer, or even a sledge hammer.


Tamping tool, eg an asphalt tamper. Again, it's just a heavy tool that you can drop onto your nut without putting much effort into aiming it. Probably best to put a tea towel on top of the nut to keep the tamper from coming into direct contact with it (especially if you've previously used the tamper on asphalt).


Use a container to hold the chestnut in place instead of your hand. Of course, you don't want to use a dish that could be damaged by the hammer, which probably rules out most of the things in your kitchen. An old square metal baking pan (not glass or ceramic), 8x8 or 9x9 inches, would work well. Just make sure it's one you don't mind ruining, because you will hit the rim occasionally. If you don't have a spare pan, you can certainly get one for cheap at a second-hand store. Put a dishcloth or thin towel in the bottom of the pan to stop the chestnut from rolling. Another option is to arrange some books or boards in the right shape, then cover them with a clean tea towel. This will help with either the dropped hammer method, or just swinging a hammer like you were hitting a nail.

Large wooden mortar and pestle. The mortar is made from a section of a log, and the pestle is the length of a broom handle, but thicker. The pestle should be heavy enough that all you have to do is drop it onto the nut, rather than swing it. So the weight of the tool does most of the work. This is a DIY project, because mortars and pestles this size don't seem to be readily available for purchase. Here's a super labor intensive tutorial for making one yourself by burning out the middle of the log with hot coals. If you're handy, you could make one a lot faster using modern tools.


Another option is to put the chestnut between two boards, and hit the top board with the hammer. This will take a bit more force than hitting the chestnut alone, but it eliminates the risk of smashing your fingers, and the target is bigger so you don't have to aim as carefully. Put a towel on the bottom board, put the chestnut on the towel, fold the towel over it, and put the top board on. The towel should hold the chestnut in place before you smash it, and stops the pieces from escaping afterwards. Cutting a small divot in each board would help.

Put the chestnuts in a cloth sack, eg a pillowcase with a drawstring added. (This will probably ruin the pillowcase, so don't use the one that matches the good sheets.) Sandwich the sack between two pieces of plywood. Drive your car over it. (I have no idea if this will actually work, but if it does, it will save you a lot of labor, so it seems worth trying.)

Heavy duty nut cracker. This one is designed for black walnuts, which have a ridiculously thick shell.