I am in the process of building a .303 double rifle using a shotgun as a donor. The shotgun has the double under lugs, gusseted root, greener cross bolt and hidden third fastener (on the tang)and pesky sideclips. I am making a monoblock now as I want to retain the shotgun barrels. I have photographed the process as I went along altho I am now waiting for barrels which will be 4-6 weeks.
I would post the photos and explaination of what I have done up to now if anyone is interested. It should be noted that this is the first double I have built and the second monoblock I started, (first one is in the scrap). The block has been fitted to the action and all I lack is fitting the cross bolt prior to installing barrels and shaping.
There are quite a few photos. I am following Ellis Brown's book and also using information from this site. (Ronvella's post was very helpful so far).