Ed Huntress wrote:

The sounds like the "BB cap" round that was popular in the 1930s. They fired
a round ball (unlike the slightly larger "CB cap") and had a primer-only
charge. However, I don't know if the primer was the same size as a standard
.22 round, or maybe had a bit more stuff.

I've got some of those. I pulled the ball on one to look at
the primer charge. Didn't do a side by side with a regular
..22 round, but my impression is there's a bit more primer.

Jon