Ditto on contacting Major. He did write a very complete article on this which takes all the guesswork out of loading brass 12 gauge BP ammo. I have seen him shoot nothing but brass 12 gauge ammo during a match and he does it very well. Durn smooth. He generously loaned me some of his ammo to try and even with just the four or six shots that I tried I was addicted. There is something very unique about 12 gauge BP brass ammo through a hammered double. It does feel different pulling it from your belt and loading it as opposed to regular 12 gauge ammo.
Blue Mesa is another shooter whom I have seen use nothing but brass shotgun ammo. I believe he has been doing this for quite some time and he is very very good with it.
Brass 12 gauge ammo certainly looks pretty darn cool in your shotgun belt; it grabs a lot of attention. Reloading it is just a little bit more time consuming but apparently well worth the effort.