Pull a bullet and check before you assume Underwood is using 800X, it'll be pretty obvious because 800x is made of huge flakes. There was a company using 800X long ago, but last I checked (admittedly it's been a while) I don't think Underwood was. I think I remember they were using AA9 or something close to it.
AA9 will do everything 800x can in the 10mm anyway, with safer pressure and it meters easier. I have no reason to bother with measuring cornflakes (800x) for my 10mm loads.
