The AR design is mature. Decades of maturing. Parts are everywhere and in a bewildering variety of types and manufacturers. And the whole upper/lower feature of the AR makes it super flexible. Since ATF says the lower is "the gun," you can buy or build as many other uppers as you want without an FFL getting involved. You can have a dedicated .22 LR upper. A 9mm/.357 SIG/10mm upper. 16" M4 style upper. A2 style upper. 24" varmint upper. 18" 6.8 SPC upper. 20" 6.5 Grendel upper. .458 SOCOM upper. On and on and on. Each with their own dedicated sighting system all zeroed for that specific upper. The AR-15 is the T/C Contender of the black rifle world! And with all the aftermarket parts, it is the Glock of the black rifle world as well!
One question...I thought Colt used a different diameter of pins so that you could only use Colt uppers and lowers...is this correct or am I confusing with soemthing else.