For a REALLY BIG coil like this one,
And I were doing one at a time, I might make a metal (steel) insert,
Use a chamber reamer for the neck of the case,
A chamber cut off from an old rifle barrel works exceptionally well for this task, but not everyone has a specific caliber scrap barrel laying around.
This would heat the metal plug (Mass), and transfer heat to the case neck when you dropped one in.
Depending on voltage input, and your timing in removing the case, you could control the temp of that metal plug VERY precisely,
Allowing you to get a very precise heat transfer to the case...
This shows an old military thin barrel chamber stump,
It will need to be cut off at the threaded end until the neck & shoulder are all that remain from the chamber,
But it makes for an easy way to 'Plug' heat/anneal the cases.
Keep in mind that barrel piece will run about 750 degrees F. so you REALLY won't want to touch it!