Martin,
I really like your idea about building a capegun, as a matter of fact, I have considered doing one for a long time. This is just my personal preferrence, but in answer to your question about what gauge to use, I would go with the 20 gauge. I think it is large enough to bring down most of the game that you would encounter while hunting with a cape gun and I also feel that it would look better next to the smaller rifle caliber barrles that you mentioned. I feel that the closer they are to the same diameter, the more pleasing they look. I realise that this is purely cosmetic, but I do find that I somehow shoot better with a gun that looks good. As far as the rifle caliber goes, they are all good choices, I would go with the one that is the easiest to find ammo for in your area. And, if you decide to use a salvaged military barrel, be sure it is in perfect condition inside. Many aren't, and building one of these guns is a lot of work, you should not use less than perfect barrels to do one. Good luck with your project. Bob H.