You may not know this, but the “AR” part of AR-15 comes from ArmaLite. Eugene Stoner, the inventor of the M16, AR-15 platform, was the chief engineer at ArmaLite in 1955 when his first design the AR-10 went head to head and lost to the M-14 as a replacement for the M1 Garand.
Today that AR-10 in .308 Winchester/7.62 NATO design lives on at ArmaLite and for SHOT this year they released the only A2 service rifle configuration available in the US today.
I have a friend who always reminds me that you can only shoot one gun at a time. That means that we all should be thinking in terms of a “one gun solution,” no matter how many guns we have in the safe. And bringing firepower to bear is how I think of what should govern the choice in that one gun that you grab in the middle of the night, or in a regional catastrophic disaster, or of course, if all of your neighbors turn into zombies and you and Woody Harrelson have to make it out alive.
Lights and red dot/green dot sights all need batteries. And in my experience, unless you use them regularly, even lithium batteries die over the course of several months in almost all devices. As an electrical circuit hobbies I can tell you that it is very difficult to make a circuit that has no residual drain on a battery, however small, that will eventually leave your device dead. If your “one gun” is going to sit in the safe indefinitely, waiting for it to be needed, the months can tick off quickly without you ever realizing it. And if the time ever comes for you to take that gun out of the safe to bring force to bear, you may be left searching for an allen key go get that sight off your rifle so you can aim it effectively with the iron sights, because your read dot is dead and nobody showed up at work in Wal-Mart today for you to buy batteries.
For that reason I generally suggest a simple and electricity free firearm as a one gun, “come the day” weapon. When it comes to caliber, that is a whole other discussion, but the AR-10 is 7.62 NATO (.308 Win.), not .556 NATO (.223 Rem.) like common AR-15s and US service rifles. As a simple gun that has a battle proven design, with a cartridge that can punch through ½ inch of pig iron and still keep going, you won’t find a more effective weapon than an AR-10. This A2 configuration is a matter of preference, so if you like it, ArmaLite, the original inventor of the AR, now makes it.