I was surprised, and a bit disapointed when I checked the throats on the 16-3. Surprised that they seemed consistent and disapointed that they weren't 312 to 313. This was due to the number of molds that cast those diameter. It would seem to limit the ones that would be useful to me. I am finding out that many of those molds cast oversize bullets, and (usually) everything is good in the world!
I guess I sort of expected the 631 to be consistent, as I figured by then, S&W had gone to CDC machines. But still it is good!
I think I were I in your situation, I would try going with the larger diameter, and see how they work. My experience is that as long as they will chamber, you may find them to be quite accurate, based on experience gained during my recent battles with a 1988 model 625, and a 625 Mountain Gun. But that is a long tale when more time is available for the telling.....
Dan P