I have several after market barrels. the only conversion barrel I have is a KKM for gen3 g21 to .45 super. I mic'ed Underwood 45 super 185 gr GD, 45 acp +p and 45 acp after shot in the KKM barrel. no base expansion.
Not sure that KKM makes .357 Sig conversion but if so I think they would do it right
btw underwood 185's 45 super are pretty dang quick.
Texas, I luv u