.40S&W to .357Sig would seem be the more compelling conversion route because the magazines can remain the same.
M&P barrels, including the Shield, have a slight bell at the end that stabilizes the front of the barrel in the slide when in battery. The bell on the "c" barrel would be in front of the slide if inserted into the Shield.What I am seeking is, aside from the length, is it the same dimensions as one for other models? The extra half inch of length is a bonus