I know it's not exactly gunsmithing, but what is the best way to clamp a cartridge case for drilling & tapping for a breech seater?

I've drilled them on my drill press before but often they'll end up spinning.

I need to change a case on a Weber breech seater.  We will do the work on my brothers mill rather than my drill press and el cheapo vice.

thanks,   

Chris.

A lathe makes it a lot easier and guarantees concentricity.

You can drill a hole in a piece of wood like oak that is case size and then cut a ‘V’ notch to the hole from one edge. Insert a case and place the wood into a vise that will close the ‘V’ clamping the case tight. Or spend a few thousand for a lathe or two. 

Jack

try this.  The left edge is shorter than the right one with the 'V"

I assumed the correct answer was with a lathe.  I would love to have a small lathe but don't really have anywhere to put it.  I will try Jack's technique before forking out the money for a lathe.  Oh and these are 32-40 cases.

Chris.

Do those little lathes have the capability of cutting a variety of small threads?  I've never looked seriously into them.  Other than size, do they have a many limitations?

If he has a mill there is a good chance he has a V-block, that would be a good way to hold a case to drill and tap it. 
As far as the small lathes go, I have a 7X14 and it has done a lot, a very lot of work for me. The nice thing about the change gear lathes is the wide range of threads you can cut. When you get into some of the odd ball threads that you find on some old reloading stuff they come in handy. Lyman seems to have had a thing for 30 threads to the inch, kind of hard to find a lathe to cut that thread on lathes with the quick change gear box. Yea you will invest a chunk of change in tooling but it will eventually pay off in the long run. 
Sam

I had to deepen the primer pockets on a box of cases.
A 5-C collet blank, drilled using the chambering reamer, made an accurate holding fixture.
Otto