The Balloon head brass is on the smaller side of other brass I have. It will go into the shell holder about 75% of the way and stop. My FC brass has a .510 rim and it will slip right in to the back of the sh, vs .506ish for the bh brass.
It's the lack of a groove at the rim stopping it from seating in the sh. I couldn't get the inside measurement with my dial calipers but I can see that the way they cut that sh is angled inside and it needs that relief cut. There's plenty of play up and down. The angled inside edge on the sh fits nicely into the groove on other brass.
I need an extra sh anyway.
