You may be right about some cases being thicker- say .375 Win., but I don't know there. Almost all of my cases are 'real' .32-40 or made from .32 spl. or .30-30. Most of my cases (when I bother to check thickness) seem to be similar necks of just under .010 to slightly over. I don't really check often because my chamber is "generous" so most anything fits as long as I stick to bullets of .323 or so. That's for fixed. For breech seating, I just put a tiny flare on and let 'em go to whatever size they end up when shot. 
When I do my conversion I've never annealed. Sometimes afte a lot of shooting, I do it, but not always. Vall has a lot of converted cases, and he may have some other insight here on thickness or uniformity, even annealing.
Further, most of the old brass is more consistent in weight than modern made. Exception is a small batch of .30 American that Terry Buffum gave me- it was the most uniform brass I may have ever owned, but it didn't end up shooting any differently than mill run.   
In over thirty five years I never bothered to do a chamber cast, and don't see a need at this late date. I figured out what it likes by shooting a lot of stuff in all that time. Plus, the advice of the old guy I got if from and an old mentor who was more savvy about cast bullets than I'll ever be. 
Since we've talked about resizing cases to become .32-40s a lot of times now, I figure our discussion might also include other aspects of the nature of the final product. John's notion about thickness is worth talking about, since it often gets little talk. I'm still curious about the chamber's effect on case life. And, to all of you out there, what has caused the most case loss in the converted brass you've had?

Before some start to think this is a really big deal over the past seventeen years I have only separated very few and only in the neighborhood of four or so give or take and they were well used. But the conversation has also been very interesting, quite friendly and some are using parent cases I would have not thought about using so it's been very knowledgeable as well and to me continues to be so.

JLouis

I just noted a curious thing- maybe. 
Using Starline .38-55 cases, gunlaker, back in post #16 says he'd had a few separations and thought it may be thin cases. 
Then, using Starline .38-55 cases, just above, Ranch13 in post #33 is having trouble because the brass is too thick.
Either the manufacturer changed something or the technique to reform is quite different. So, maybe the guys that used the Starline for forming can help out here.
I only did a very few from any (old).38-55 because I never saw many back when I first started doing this caliber. All that stuff was obsolete, and didn't show up much in usable form. Thats why my old "real" cases are out of business companies, or headstamps. Now, the only .38-55 that I acquire goes to a friend to feed his old Model 1894.

Could be different- like I mentioned, but did those guys make the two different lengths of that caliber? If they did, maybe that's part of the change. Dunno, not having any of their stuff. 
I wasn't criticizing either of you guys, just pointing out something I'd noticed that is curious. 
You have some old Western cases too, huh? I lucked into a hundred 'real' ones way back in the dark ages. The old CIL cases came with the rifle when I got it back in '79-80. The rifle came from upper midwest, near Canada, and I figured those case showed up there more than here. 'Nuther dunno from a guy that wings it as often as not.

Only problem I am aware of with the 38-55 resized cases is from one in our group who had problems with them sticking in the chamber after a few firings and just requiring re-sizing but not sure of the brand.

JLouis

I've never had a single stuck case in .32-40 either. Perhaps your guy over annealed them? I can't see much happening to a breech seated case unless the powder charge is up into the ozone.
Possible exception from too little powder folding a mouth that never got sealed.
Some of you out there must have ideas here, let's hear 'em 

Interesting Don that your Starline .38-55 brass is thick.  I have piles of the stuff that I bought maybe 8 or so years ago and it's all about 0.0075" thick at the case mouth.  All of the cases I've had, long or short Starline are the same thin necks.  I would actually prefer that they were thicker, but I don't really shoot fixed ammo in .38-55 and only rarely do it in .32-40. 

The Starline case I lost to stretching weren't reformed to .32-40, but were still .38-55's.  They were loaded with FFFg and now that I think of it, some were Old Eynesford 1.5 which also ripped them in half .   Swiss 1.5 or plain Goex powders in Fg and FFg were a lot friendlier.   Interestingly I've had some really excellent results with plain old cheap Goex Fg in .38-55 and .32-40.   

I have had one .32-40 case failure with BP and it was the kind usually associated with smokeless.  The case mouth had crumpled as the pressure got behind the case before it had expanded and sealed.   

Chris.

Chris if you can get the starline  stuff back home with you I'ld bring it to Worland. It serves me no purpose.