I just got a new brass bag mould from Dixie and now I understand.
The one I got seven or eight years ago was cut for a .610 ball. It throws a decent ball, but I wouldn't want to try to use it for everyday casting. I wouldn't want to use the steel one I got from Rapine either. They heat up quickly and no matter how well you insulate the handles they get HOT.
As for the new one, its cut for .350 and it dosen't look to be as well finished. I need to file off flash from around the fill hole and who knows what the ball will like it thows. The mould itself is very small compaired to the old one I have.
Anyone finding a GOOD brass bag mould send me a PM I'd like to have one for my pistols (.440) and one for my trade gun (.600) even it they are just for show.