Bailey,
I'll be interested to see the results that you are able to achieve. Virtually everyone that I've read and talked to, with experience in rust bluing, says that to go beyond 320 grit is counter-productive. Brownell's own instructions say that to go finer may leave the steel with a surface that the acids are not able to "bite". Certainly, so far, I can see a difference between the barrels, the ribs, and the scope bases, since each steel has a different chemistry. It's been my experience that, with the Herter's Belgian Blue, this difference often, but not always, disappears as the number of coats/cycles increases.

At Oscar Gaddy's suggestion, I've played with etching the surface of "difficult" steels with a ferric chloride solution. It leaves a frosted surface and definitely allows the first coats to grab the steel better. I decided to follow Brownell's instructions to the letter on this first rust blue attempt and see how it works out.