1) Without a lathe, how will you thread the barrels for the mono bloc? Can't see Walther doing it really.
2) Before worrying about recoil what is that 16 Guage proofed at, that will determine cartridge choice to some extent. Also the C-C on the firing fins.
3) With due defference to 3sixbits, I dont think that it is going to be very easy to ream a 3" plus long chamber by hand. I know how hard my L5 was working in backgear and the cut pressure. IMHO to would get an ovaled chamber. Then theres trimming and crowning etc.

Lathe and no milling machine yes, the other way round I don't belive so.

Regards