Thanks, guys,
Now that I recall, the spent paper hulls were distinctly brittle. As I said, they weren't very old, but maybe they do get brittle with a few summers and winters on them. Add some possible stickiness in the chambers (I will confess to not having swabed out the barrels as they came from a relatively warm gunroom to the very cold outside and we may have the explanation. (Holmes, how do you do it?)
P.S. Steve, I went inside for a warming drink as fast as my little legs would carry me.