My neighbor was impressed with my 45 acp shot shells using 308 cases that were cut down and then necking them down using 40 cal dies and asked me if I could make him some 40 cal shot shells. So here goes... the 40 cal case head spaces off the case neck so I believe that eliminates trying to neck down a 40 cal to say 38cal or 9mm because I believe the case would go too far into the chamber. So having some 10 mm cases I am wondering why can't I simply neck these down to 9mm for a distance of say .125 which in theory would allow the case which now would approximate the same length on the straight wall section as a 40 cal casing. This would be a two fold blessing as it should allow the case to headspace the same as the 40 cal case and its additional length should in my opinion help to aid in chambering a round.
What I have learned so far...
Trying to neck down a 40 cal shell won't work because it won't head space correctly.
Annealing is certainly necessary in order to even remotely neck down the case because otherwise the neck will crush in the process of trying to neck it down.
There have been some commercial 40 cal shot shells made with a straight case but they won't feed from a magazine and the individual shells have to be dropped into the chamber.
I'll try to link a picture of what I have so far. I made several 10mm cases that I necked down to 9mm that should head space correctly in a 40 caliber.