That sized round ball will give accurate measure as the resulting 'flats' will be plenty long enough - if the measurer is accurate.
Be careful you don't compress the corners of the groove impressions when measuring, as they can compress easily, giving a small measurement. Smaller is better than larger.

Mike - your post said the 'bore' was between .403" and .404" - I take that to mean the 'groove' diameter was between those two measurements.

Lee Precision will make you bullet reducing (sizing) dies for around $25.00 each. mperrial die wax or STOS does a good job for lubing bullets for reducing. I am currently sizing .375" down to .367" in one pass in a die I cobbled, with barely more pressure than FL sizing a large case. those two lubes work well, but some lubes aren't a high enough pressure lube and might stick a bullet.