Briley rates their factory replacement chokes for steel only up to Improved Modified.
http://www.briley.com/2009/chokefaq.html
As you can see from this table, Improved Modified can be considered to be equivalent to Extra Full when used with steel.
http://www.briley.com/2009/lead.html
There is nothing to be gained from using an Extra Full choke with steel and you run the risk of damaging the choke and barrel. Don't do it!
According to the charts supplied here, I should be going the other way. First solid bit of info gained so far. Thanks.


















































