I'm curious where you looked that showed that the .357 actually slowed down after passing 16" in the barrel.....
I've never seen that with any round. Now, some might only gain another 50fps (or even less, possibly) with another 2-6inches of barrel, but it is still a gain. I would be really interested in seeing valid data showing a bullet actually slowing down due to a longer barrel...
Sure, there is drag, but all I have ever seen in nearly 50 years of looking at these things is data that shows a reduction in the rate of velocity increase. If there is new data out there, showing an actual decrease, I'd love to see it.
And don't confuse a lower muzzle velocity from a longer barrel with a decrease in bullet speed due to a longer barrel, as there are other factors than just length that affect velocity. An extreme case, but I have seen a 4" revolver shoot "faster" than an 8" one. Not because the longer barrel slowed down the bullet due to its length, but because the specific barrels tested were "slow" and "fast" beacuse of bullet/bore fit in those individual barrels.
The only way I can see to proove a longer barrel "slowed down" the bullet due to drag (and discount any additional factors) would be to take a long barrel, clock bullet speed, and then start cutting it back, firing the same ammo, until you found a length where the velocity was actually higher than the MV at the start. Personally, I doubt that will occur.