Pretty serious underbore--like almost .020 under the standard .550--assuming the bores have not been enlarged much since it was proofed as a 31.
As for the popularity of the 16, the A5 didn't appear in Sweet 16 configuration until 1937. By that time, American makers had already produced thousands of domestic 16ga side by sides (over 25,000 from Fox alone, and likely more than that each from Parker, Elsie, and Ithaca--not to mention the hardware store brands). And by that time, most American 16's (except the Elsies) were chambered for 2 3/4" shells, while the prewar Sweeties were still 2 9/16".
