Another thing about modern made 66 and 15 bobbins is there is little consistency. In the same package some will be narrow at the rims like Rodney said, some will be rough, some will wobble on the shaft (I've had some that were so out of speck you couldn't wind them on the machine).
I've got to the point I go for all the old vintage bobbins I can get. The only place I get new bobbins at is Sew-Classic.
Joe