As I did say I don't anneal cartridge cases so I'm no expert in saying what exactly to look for to determine what degree a case may be over annealed.
Having done metallurgy many years ago gave some experience with annealing different metals and non ferous metals which behave in different ways.
I believe a cartridge case is in general a composition of 70% copper and 30% zinc but may vary between manufacturers., there is an ideal temperature at which brass will become anealed. Go over that point and there are a number of things that can happen, it will melt, it will become too soft and will loose it's property of springing back to shape and you can also break down the grain structure of the copper / zinc mixture. I do believe you can also deplete some of the zinc content permanently which will basically turn it back to copper, not brass. Some of these tests can only be proven under microscopic examination.
So, at what stage is it over annealed. It got too hot. There is a not so complicated test that can be done with a pair of vice grips starting off with a brand new case and set them just to apply pressure to the case neck enough to very slightly deform the neck and when released the brass should spring back to it's original shape. A properly annealed case should basically do the same but if it is over annealed it will stay deformed as it's too soft and has lost it's spring back properties. This is required with a cartridge case to one provide tension to the seated bullet, second allow the neck to expand and spring back so it can be extracted from the rifle chamber easily.
Over annealing can also soften more than the case neck flowing down to the actual case body creating a weak spot which can lead to the sections breaking apart. Also an over annealed case when neck sizing can force the neck back into the case body efectively crushing the case shoulder since it has lost it's strength.
Depending on the amount of over annealing can cause all or any of these points and more. There are many technical acticles written on not only cartridge case annealing but annealing non ferous metals and the methods to use plus the signs that follow if it's not done correctly. I would suggest that anyone that wants to do cartridge case annealing search these articles out and spend the time absorbing the information. I will be annealing cartridge cases in the near future so I will be reading those articles in more depth taking notes but more so experimenting first hand, especially before attempting to anneal any of my more expensive cases.
A basic test, I would try the vice grip system first. If I thought a case was over annealed slightly then I would see what happens when attempting to size the full neck, not just part of it and if it didn't damage the case then I would try seating a bullet and take a close note of the feel of what pressure if any is required to seat the bullet. Naturally my In-Line Seating Die with Arbor Press would be the best to use as it gives a very fine feel of the seating pressure required. If there is no pressure required then the bullet may even fall out or at least be pulled out by hand easily. I will say that some of my target rounds are indeed like that by purpose as with some I use very little neck tension.
I have heard of a badly over annealed neck being able to be distorted with just one's fingers and it didn't spring back.
Remember cartridge spring back is required in order to remove the fired cartridge case from the chamber.
Firing a brass cartridge case in general doesn't work harden the brass to any major degree as the amount of expansion and contraction is too small to be effective. Chambers that have very loose tollerances may work the brass to a greater degree, Bit like bumping case shoulders way too far back and/or having way too much chamber head space that will hasten a much more dangerous situation like head seperation. Neck seperation at any point is really not as dangerous, just a real pain when you have to extarct the broken off neck out of the chamber without damaging the chamber finish. Here there is also a slight chance that that broken neck will be forced partly into the bore, perhaps causing a huge spike in chamber pressure before the bullet is hopefully passing down the bore.
On a further point there is a lot of debate about annealing in general being of any real advantage or having any effect on overal accuracy. Some very well known target shoots don't bother at all with case annealing, shoot their cases for a determined number of times and then replace them with new. Some also anneal their cases after every firing with the idea that it provides far better and consistant neck tension of each round fired.
There seems to be no doubt that case annealing extends the effective life of cartridge cases.
Then there is inconsistant annealing of say one side of the neck and not the whole diameter. I would be treating an over annealed case with caution without knowing just how much it was over annealed. That case has a lot of work to do subjected to say up to 60,000psi or more in the chamber.
Sorry, I could not provide a short simple answer. Perhaps someone with years of experience case annealing can.
Here is just one article that I just found that explains a little of the tips and traps of case annealing. Seems a good read if you get through it all but it's just an article....
http://www.6mmbr.com/annealing.html
