The midlength gas system was borne from the M4A1 and M4, man, plus the application of the 1994 AWB. I was issued an M4A1 in Jan 1995. It's all about distance from barrel end to distance from gas port, and it started with the M16. That's the whole reason the M4 is what it is, and why carbine-length gas systems are what they are - on the 14.5" barrel. It all comes from the M16, 20" barrel, rifle gas system, and THOSE lengths, and it's a direct measurement - for the M4. M4 feedramps came after that one, and those were full-auto derived, due to the shortened barrel, over the 20" M16.
Midlength gas came from the ban era, when barrels HAD to be 16". The M4 carbine with carbine gas, the civilian mandate of 16" of barrel - that equals midlength gas systems, almost... M16 with rifle gas, M4 with carbine gas - now you have the 16" requirement on civilians... bump the gas tube length. Midlength gas was born...
Combine MORE worlds nowadays - 14.5s with midlength gas are FUCKIN SWEET!
That's how all that $hit went down. No joke, no lie...