Someone I know is playing with the idea of making a DIY derringer, 9mm or so. He asked me if it's possible to use a 1/2" or 3/4" bolt for the barrel, drilling it out and rifling it strictly for legal reasons.

Is this a totally stupid idea, or only partially stupid?

What grade bolt?

High grade bolts are made from alloys with higher tensile strength than 4140 or 4150, but are also already hardened and tempered, so would probably need to be annealed to be worked, then retreated for best outcomes. If you just bulled through with carbide tooling, the higher grade bolt material is strong enough, but might be too brittle - needs testing.

Schedule 40 pipe and lower grade bolts will typically have lower grade alloys with lower tensile strengths than traditional barrel steels, but on lower pressure pistol rounds, will probably be adequate - though I'd do quite a bit of testing before I'd trust firing one myself.

Why re invent the wheel? Click here.

Simple answer is "if there's enough material supporting the chamber, yes".  

9mm in a very small light weight derringer will suck.  So take care of both problems by using heavy enough stock for the barrel and frame to support the chamber and mitigate recoil.  

If one were only going to do something like this once, by all means, buy a prechambered barrel, liner or adapter.  But for some, it won't be just once and we enjoy the "built not bought" projects.

My current build project  and 9mm is on the list.  Already have the rifling button and chamber reamer.