13 years? Go back to the late 80s.
Service rifle freefloat barrels were made by a guy in Plano, Texas. He welded your front handguard cap and a custom sling swivel mount to a metal tube and welded the tube to your barrel nut. A2 handguards were then modified to fit.
Beginning in the early 90s, Milazzo made the only two stage trigger, Kreiger distributed it.
Sierra released their 80 grain HPBT and now you had a 600 yard rifle.
A popular cottage industry mod was a threaded rear sight into which you screwed apertures of various sizes depending on personal preference and lighting conditions. The square A2 front sight was ground to a width matching the MOA of NRA high power rifle bullseyes.
If you get the impression most advancements were for NRA service rifle use, you’d be right. We were the only folks buying ARs in the 80s. Even then the AR was vastly outnumber by M1s and M1As on the firing line. Ridiculed as a “mouse gun” until everything came together and they began winning matches. Now you had a target rifle that just required a torque wrench to assemble. None of that artsy shit like glass bedding wood stocks or hand lapping rear sights on M1s, and M1As, or eliminating op-rod bind as the barrel heated.
The first commercially available freefloat tube was an aluminum extrusion with the barrel nut machined into the end. You used a strap wrench to tighten it. Don’t twist it, it didn’t have an upper interface and may come loose. The front sight post was cut off.
Flat tops were A1 uppers with the handle machined off and a Weaver mount (Pic rail V1.0) screwed on.
Those last two developments were for killing Prairie Dogs en mass. Otherwise, any “serious” shooting was done with a bolt rifle. You were a “fool” if you expected accuracy from an AR.