From what I read somewhere the actual bullet diameter for the 38 special and the 357 is actually .357 inches. The nine millimeter works out to .354 inches. I see lots of ammo that uses 125 grain hollowpoints in both the 38 special/357 and 9mm. Is the bullet manufacturer actually casting two different diameter bullets in the same weight?
I/We have a ruger GP100 and yes it can fire both the .357 and .38 rounds, same diameter. When we shoot both at the range we always fire the .357 first then finish with the .38. It doesn't take long for a ridge to form and make it hard to get the .357 fully seated.
Extra attention when cleaning, always. My pinky is small enough to feel for the slightest ridge and I make sure every bit of it is removed.