You may want to look at E. Brown's book Building Double Rifles on Shotgun Actions. I took gunsmthing class from him, covering same subject, prior to his writing book (which I also read), and learned, among other things, that he built his first .450 nito double rifle on 16 gauge German action, only to say that he would never again use a 16 gauge action for that particular caliber, because it simply doesn't make for a heavy enough action, to balance out with the weight of the barrels, etc., giving a gun that is too heavy in the muzzle area. Too, making such a gun lighter overall, only makes for added felt recoil effects. In my double rifle building work, I too have found that I agree fully with Mr. Brown's thoughts about this matter. I would not use a 16 gauge for any rifle in the .450/.470 size, and maybe not even for the .400, but much prefer results from using a 12 gauge, or even maybe a nice appropriatly sized 10 gauge action. Best, Jack Sullivan