One thing about it, don't do it if you're not prepared to just junk your factory chokes and make a trip over to Briley: that happens a lot when people start patterning guns. I know I did way back when..
Also: it's not the gun you're really patterning: it's the choke: if it's choked right, you can get a decent pattern out of a piece of steel pipe... Thus you need to make sure you do it with any choke you plan to ever use..
I may have a good explanation for that......I definitely have a substantial amount of my money tied up in Briley chokes- they are excellent, and I know Stever personally down there. But I would not give a blanket agreement with your statement about junking your factory chokes.
In my previous post here when I did all of that pattern work with my 870, I used only the factory Remington chokes, and got some excellent patterns. Now I will say that the beretta chokes I have gotten with their guns have been the most irregular of any manufacturer I have ever seen, and have always replaced them with briley's.