Consider
Port pressure. Original 20" barrel, straight gas tube, buffer x and spring tension x, all calculated to provide reliable function with gas port pressure x (provided by ammo loaded with xxx)
First M16 reliability issues show up after change in ammo spec (powder type), which changes gas port pressure.
Now, take the carbine, 16" bbl. gas port pressure is different from rifle, due to being taken at a different position on the barrel. Straight tube still works, with adjustment to buffer and spring tension.
Then look at the really short barrels (M4). Gas is tapped much closer to the chamber. Buffer/spring adjustment isn't quite enough, so the "pigtail" gas tube is developed, to "mimic" tapping the gas further from the chamber. Without the balance from the buffer/spring combo proper for the pigtail tube, functioning becomes erratic.
The real question is, (if) your carbine was working properly, why did you change it?