Definitely not excess lube (a common cause).  And I've tried Imperial, great stuff, but for heavy duty case forming anhydrous lanolin beats it hands down. Besides, it doesn't happen every time and they're all lubed the same.  The pattern isn't uniform either, causing me to suspect it's something variable with the brass.

The problem I've had with short brass, and it's too long a story to relate in detail, has to do with copper jackets or lead swaging into the space between the case and the end of the chamber and shearing off, leaving a ring of metal.  You seem to be thinking damage to the rifle, and that wasn't the case.

Paul 

The worst case was a custom .338-06.  While it was still fairly new, and working up loads with a certain two-section jacketed bullet with exposed lead at the base, I was getting a nice little copper ring on my cleaning brush on the back stroke.  In effect, the sheared jacket material was creating a "bushing" at the front of the chamber. This was years ago, and the rifle has gone down the road.  It only happened with one brand and type of bullet.  I was using necked up .30-06 brass, so it was a bit short for the chamber.  Later I went to necking down .35 Whelen brass to avoid the short case issue.

However, it was quite an eye opener, and made me forever leery of short cases.  Since I took up shooting cast bullets, even more so.

I seem to recall reading about this phenomenon with lead bullets, but can't recall the details or source.

Paul

No, I wasn't really worrying about a damaged rifle. I only mentioned mine is still fine after way over a century because I thought you might be worried about that. No damage here. 
If John is getting much damage it's perhaps his powder choice, mine is OK with mostly 4227. Don't know what it got in its early life. 
Never had a problem with the little rings, but have heard of 'em. Throat size, angle or alloy?

John, I wasn't trying to indict you over your opinion. I was just musing while responding to the posts here. 
Mostly, I'm just here to affirm your praise of the .32-40 as a good choice.
I was curious about his troubles with the short cases. Now we all know. I haven't had the same, and my rifle- even if it isn't up to anybody's high standard, has never worsened from the use of the shorties. Just a good 'ol conversation, ya know. 

A couple of the top CBA shooters here are using CPA 32-20.  They are not picky at all according to reports from the winners circle.

When breech seating:

Why does it matter what length the case is.  All a case does is hold a measured amount of a propelent and a primer.  IF the chamber is made to fit the case or visa versa, no problem, right?

If the case is short the chamber just is now part of the case, right?


I have had 32-40, 32 miller short, 38-55... Now all I shoot is a straight tapered 357 max case, necked to 32 cal.

SO why not use a 357 mag case necked down?

All the case is used for is to hold a measured amount of powder, and a primer.

250  RKS barrel.

If someone wants to use a case that is shorter by more than just a few thousandths than what the chamber is designed for it is their choice to make. If they also feel it has no ill effects on achieving the best in accruacy that is also their right. My opinion based on my experiances all though they might not agree with others are simply just mine. Erosion does indeed take place and it is very substantial in the throat of my 32-40 but I also tend to shoot more than those who have yet to notice it as it now has over 144,000 bullets down the bore. Why someone would purposely choose to use a case much shorter than the chamber design simply goes against the correct and direct relationship of the cartridge to chamber fit as outlined in the SAAMI specs. If one feels there may or may not be any ill affects to the ultimate in match accuracy it would simply be something they would need to prove to themselves as I have done for myself. My post was not intended to be argumentive or to be criticle of CF or others I was simply sharing a bit of my personal exsperiances and my opinion and nothing more. I beleave and hope CF and others understand that intent but unfortunately and quite typical it is becoming argumentative but not by my choice. 

JLouis