Other than zip guns and muzzle loaders, what are some of the simplest gun designs with the fewest number of parts? Are there any that an average guy with some mechanical skills could make with only hand tools?

Have you looked at the JACO designs, they are simple and can be built with hand tools. With some refinement they could even be kinda decent looking. The prints can be found online for free, do a search and you should find lots of references to JACO Pistols and Rifles.

Thanks very much! That looks like a good place to start. I'm also looking at the underhammer muzzleloader locks and wondering if there is a way to adapt one to a rimfire cartridge.

I have also found a bunch of ideas in the homegunsmith.com forums.

You might look at an old M3 .45 ACP Grease Gun for inspiration, hard to make anything go bang with fewer moving parts.

True: open bolt slam fire is the simple way to do it; but don't make one with a magazine; it has to be single shot. Even with a disconnector, the BATF frowns on them. To easy to make full auto.

In Northeern Ireland, during the "troubles" the loyalist paramilitaries made square section smgs. Basically a Sten "knock off" but made from the square section steel tube used by furniture makers.

Or Google the Rhodesia War LDP pistol.

Wern't sten knockoffs made by resistance fighters during WWII In bike shops?

I have a book on building a falling block single shot, can't remember who wrote it and I am not anywhere near it at this time. Think I ordered it out of RIFLE magazine.

The rollingblock design is pretty simple