Ach! Junkwaffen.

Anneal what for what and why?

I've mostly annealed cases to fireform them to another caliber.. specifically from .303 British to .410 Enfield musket.

Oven annealing doesn't do much and it can't because you are evenly heating the whole case. If you took the whole case up to high enough temperature long enough in an oven you would soften the whole thing and ruin it.

For fireforming cartridge cases to another caliber, you soften whatever portion of the case needs to change its shape, then fire a light charge of powder with a load made of something like farina or grits. You need to take the area to be annealed up to a high enough temperature to see it turn red just a bit, and then immediately quench it. Both the heating and quenching are necessary not because I say so but because people who think they know what they are talking about say so, mainly that if you want brass to be annealed instead of turned to putty from being heated that hot, you have to quench.

Then there is neck annealing to prevent splitting... pretty much the same as above but focused more closely on the neck and shoulder.

A lot depends on what "strength" loads you will be using and the specific cartridge. My musket cases are only good for .303 Enfield conversions made for riot control in India. The .303 case head is too thick to fit an actual .410 shotgun. Used with the recommended load, they suffer very little stress and will last through many, many, firings. If you are annealing a high powered rifle round or especially a magnum round, the whole case deforms with each firing so much that while annealing may save the neck and shoulder awhile so you get some more firings, if you keep at it too long you will have case head separation instead. Annealing won't prevent that.

That's my story and I'm sticking to it.

BUT, I very much recommend that instead of taking this question to a bunch of internet commandos like us... scour the web for formal, correct, sources, not random opinions of random people. And/or better still find some published-printed sources. Pay real money for them. For some purposes to screw up might be safety-critical. For what I do I can get away with all kinds of technical malfeasance.

The only real bad life experience I can relay, is that if you ever anneal with molten lead, do NOT do what I did. If you forget to oil the case first, you have just soldered the outside of your case and will have a devil of a time removing the lead which adheres quite well to brass. In that instance the brass in question was the enormous .577/450 cartridge for the Martini Henry. You could have heard me cussing all of the way down to Slopchoppy. Second worse bad life experience is forgetting what I just did and grabbing an almost blazing hot piece of brass with my fingers. They heard me in CUBA on that one...