At one time he was using 20ga Browning SXS actions for conversion. This action has a double underbite lock, he was adding a Greener style crossbolt for a third fastener. The third fastener makes the action much stronger for rifle use.
There is a book out detailing the conversion of double shotguns to rifles. It is quite a process. I have seen three conversions done by a chap on ON; a .22rf, a .303 and a .450, iirc. Very impressive work, very beautiful rifles.