How to shell macadamias without a special tool?

I've used C-clamps before. You set the macademia in it, then tighten it down 'til it cracks.

You can keep a gloved hand around the nut while turning the screw with the other hand to avoid the risk of flying shell bits. (but you should be turning slow enough that it doesn't crack explosively)

It does take some time, but it's less than a minute per nut. It goes faster when they're all similarly sized, so you're not adjusting the clamp signicantly to fit the next one.

Macadamia shells take somewhere around 300 pounds per square inch to crack, which is an awful lot, but doable. Roasting may, according to some, make the shell more brittle, how long and how hot I do not know.

Using a hammer is of course one way, but as you say you can end up smashing it to pieces. I would try your hammer method again this time with a chisel. The chisel will focus the energy of the hammer blow so you can hit it much softer and more precisely. You could also try a vise as you can put loads of pressure on with high mechanical advantage. That's a load of winding and unwinding though.

If you have a large locking pliers I'd try that as well. Keep on ramping up the pressure using the adjustment screw until you get just enough to crack it.

Whatever of these methods you use please please please use at least eye protection! A full face shield may be better.

Opening macadamia nuts is trivially easy. Just place the nut in the jaws of big vise grip pliers and wrap it with a napkin. Attach another big pair of vise grips TIGHTLY to the adjusting screw of the first and gradually crank down, holding the wide part of the first vice grip with the nut wrapped in a napkin. Crack! One perfectly opened Macadamia in less than 10 seconds!

Even inexpensive locking pliers from Harbor Freight Tools work well for this purpose. Name brand Vise-Grips may be overkill.

Check this link, snapguide . It shows a method that could easily be replicated, perhaps to crack several at one time. As I've read elsewhere, the trick to cracking the shell without destroying the nut is to place the nut in a divot. (I would use something slightly deeper than shown in the link.)

I found that boiling the nuts for 15 minutes softens the shell If you make a hole in a piece of foam and push the nut into the middle, place on concrete and hammer the top.

Perhaps a good mortar and pestle can do the trick.

This would be closely related to your hammer and divot-in-concrete method, especially if using a good strong mortar and pestle (mine is heavy granite). However, the benefits of this are that it is easier to handle and move (you can keep it at a workable height), you can control the force more precisely (several lighter taps to fracture, then tease the shell apart, mean less chance of the kernel breaking), the nut is more contained in a deeper bowl, and so won't flee if the strike is off-center (again privileging several lighter taps, preferably at different points on the shell).

In case I was unclear, the emphasis is on several sharp controlled taps to fracture the shell, not great strikes which might injure the kernel, or more importantly, yourself. Let the weight and hardness of the pestle do the work. The force of the strike, and the nut, will be contained in the bowl of the mortar.

Method is basically drop a nut in the bowl, thwap at it a few times until you hear a crack, move on to the next. If it won't tease apart after, drop it back in for a few more thwacks. I've shelled different kinds of nuts this way, though not macadamias specifically, but I see no reason it should not work if your mortar and pestle are of a decent size and weight.