I cast my own hbwc's for the 38spl's & 357's. Have for decades using a single cavity lyman mold. Did a lot of swaged hbwc's also. Several years ago I bought this 4-cavity mold.
Life is good
If I was a gambling man I'd say:
Too hot of a load
Bullets sized too small
The wrong expander was used on the brass and the bullets are being sized down even more when seated.

Life is good
If I was a gambling man I'd say:
Too hot of a load
Bullets sized too small
The wrong expander was used on the brass and the bullets are being sized down even more when seated.