I have that book as well.
No preference on caliber. Something potent, but I have nothing to prove by trying to go big. I have two custom rifle builds to accomplish to complete school. One will be using a small ring Mauser I have on hand, and because itís a small ring they only permit smaller caliber builds. Fine with me, so we decided on .300 Blackout. Next will be a large ring Mauser, when I find one that I can afford.
The motivators for the double rifle are: because I want to, I think theyíre fantastic, I wonít likely be able to afford one otherwise, and party because school isnít keen on the idea. I would be very happy with a mid-range caliber, but since Iím not well versed in rifle calibers, I donít know what that would be yet. Iíd like it to be on the safe side for this kind of thing, and available over the counter. I reload shotshells for my old beater doubles, but donít have rifle equipment.
Anyway, it sounds fun, and incredibly difficult, so why not. I did see what looked like a good German action for this project, but Iíll miss it. Need to sell off other projects first.