400 gr Lyman Snover sized .410. Case not sized. Badger barrel, High Wall action  with DST.  Fired several test batches today but this is only one I had trouble with. 25 rounds total. I am considering getting a 40-70 SS reamer based on a .405 case as I have had these reformed 30-40 cases many years and they are no longer available. I assume I have to cut a new chamber to use them.

Wiping? Drying chamber with bore swab? How are you cleaning your brass?

40-70 2.5 inch chamber.  One wet patch after round.  Brass are washed after using and then tumbled in ground corn cobs.

Quote:
Cases can be separated when blow tubing also. To keep the chamber dry - insert the tube in a hole drilled in the rim of the case
Number of blows to moisten the foul:
* Relative Humidity below 44% - 12
* Relative Humidity above 44% - 5 to 7
Source: Bill Knight, the Mad Monk

I had some 40/60 Maynard cases from BACO made from 30/40 brass. I had not fired it in years and went to load it.  I had several separate in the sizing die. It pulled half in the die just like yours. I ended up throwing it away.

The bullet with the case forward of the separation started into the barrel and engaged the rifling, At this point the bullet with 1/2 inch of case still stuck to it separated again leaving the middle of the case started into the rifling. The bullet with the front of the case still stuck to it proceeded down the barrel like a jacketed bullet and went through the target sideways.

Even when the cases are tumble cleaned with ceramic chips, nice and shiny inside and out, separations or stretches of the case into the rifling can happen if water is left in the chamber.  I've had my trouble exclusively with paper patch bullets, which require wet wiping; whether it's because I use a blow tube or the lubrication on the bullets, the grease groove types haven't exhibited this behavior.

The principal offender for me is the .40-90 3-1/4 Sharps Straight, although I had it happen once with the .45-70.  Been luckier than you, JSB30; the cases that broke either came out of the chamber with the extractor and a cleaning brush, or the broke-off sections were carried clear out of the barrel with the bullet.  Didn't seem to hurt the barrel, but probably not all that good for it either.  "Jacketed bullets through old soft-steel barrels," and so forth.