Hi DC,
I measure the widest part best I can with a caliper. I keep it as square as I can and expect I'm close to correct. You can see how I do it in the pictures.
I don't know that I am driving any of them hard enough to lead. I shot some of the 155s w/ 6.0 231 but they are a good hard cast bullet and I would guess that load around 950-1000 fps.
I looked it over pretty good today because of the backwards bullets. I was half expecting them to melt or act funny. After 40 rounds of various loads I'd have to call the leading minor if any.
Are you talking barrel leading? The throats are more messy than the barrel but nothing to worry about. They do show more than the barrel which shows little or nothing. There is nothing in either that wont easily brush out.
Thanks
Mike