Tru-Choke is specifically made for thin wall barrels. These work great and have a lot of chokes available. You can buy the tools to do it yourself (lathe recommended) from Brownells or you can send it in to have it done. You will have to do a web search and possibly find a local smith that does tru-choke.
The downfall is that they are rated for Lead shot only due to the thin wall of the choke and of the barrel after reaming.
I am not sure if the Briley will accept steel shot in their thinwall tubes either. I have no experience with them in this area.
After S&H is figured in You are looking at around $250 for an O/U in and $150 for a semi from what I have seen.
Here is one guy that does them
http://www.gunbroker.com/auction/ViewIt ... =114604470
CPL holder since 1991, NRA Lifetime Member, DU Member, NICET certified
"For those who would give up essential liberty to purchase a little temporary safety, deserve neither Liberty nor safety.—Ben Franklin.”