Since it is (apparently) legal to build a 'copy' of someone's revolver, I am putting this in the Handguns forum.

Assuming I want to build a BP pistol, and have a drill press as my most advanced tool, how should I go about indexing the cylinder so it'll turn when the hammer is cocked?

You don't.

Revolvers are simple looking actions that are more complicated then they appear.
Samuel Colt was a true Yankee machine inventor.  When he started building his first revolvers, he had to invent the machinery that could do the complex machine operations needed.
Colt and his mechanics were forced to invent modern barrel boring and rifling equipment and modern lathes and milling machines just to make the "simple" black powder guns of the day.

Without a good lathe and milling machine and some high-end machinist skills you'll find actually making a gun from scratch is not going to work out.
In order to get a cylinder that will index requires surprisingly high precision equipment that a drill press is simply unable to do.
Among other operations, the chamber holes must be high precision bored in perfect alignment and the locking notches and the ratchet on the rear have to be very precisely located or the gun will be firing with chambers not aligned with the barrel.
There's no way a drill press can machine the ratchet, and its not precise enough to bore the chambers correctly.

The second part of cylinder rotation is the frame, hammer and "hand" that actually pushes the cylinder around to the next chamber.  The frame is a complex part that requires the ability to machine-broach square holes for the hand and hammer, the hammer and hand construction are way beyond a drill press.

I suggest buying a cheap, used black powder revolver, disassemble it and look at the required machine work needed to build it.

My advice:  Either buy a BP revolver, or if you just want to do this as a project, buy a BP revolver semi-finished kit and assemble it.

I'm a retired Master watchmaker/Colt double action revolver gunsmith.  Even with my high-order hand work and filing skills I couldn't do it with just a drill press and come up with something that would be safe to fire or even work correctly.

Yes you CAN build your own http://www.atf.gov/firearms/building_a_firearm.pdf
As stated above, building revolvers is actualy very technichal (odd, since they came before autos) Building a 1911 would be easier.

Dfariswheel's post is spot-on. His reasoning is the reason I would prefer to build a "Pepperbox" style pistol.

the Pepperbox is essentially a revolver without a barrel, but WITH  a very long cylinder. Essentially you have 6 single-shot BP barrels in one cylinder.

The reason I want to do this is to illustrate my oft-stated argument that "If a magic magnet picked up all the guns in the world on Sunday, the cops would be outgunned by Wednesday."

The cops would be out-gunned by Monday by home-made submachine guns and shotguns.

Few automatic weapons are as simple as the Mark 3 STEN gun which can be made with muffler pipe, and the sliding tube single shot shotguns made in the Philippines are nothing but two tubes.

I once saw a confiscated zip gun made from some steel tubing, a nail, and a rubber band.
I've known several buckskinners who make their own black powder and one who makes rifled barrels.
I'd guess that a pepper box pistol would be do-able, and since jungle work shops have made automatic pistols with not much more than files that's possible too.

Some home made pistols have frames and even slides made by silver soldering layers of steel plate together.

I love working on little projects like this however I have no real insight to share in this case.

I would love to hear more about how your project works out though.




 

Call (423) 496-9050. If they're still in business, that's ET CETERA L.L.C., they sell guns and kits. They had a pepper box 45/410 kit pretty cheap (also available in black powder.). At least ask for a catalog. If they're still around, the have a shitty hand stapled catalog.
ETA: Dixe gun works has a black powder pepperbox kit also.