Got the phonecall from Matt after your breakfast. Apparently, you're a somewhat decent person... Welcome aboard, again, in a different perspective now.
That 4-40 set screw is for the takedown pin - the takedown pin hole should be threaded for the 4-40, to capture the takedown spring - once the spring is trimmed (by the length of the set screw), you insert spring, capture it with the 4-40, and you never have to mess with it during disassembly. The whole purpose started with the AR15 platform, and carbine recoil systems, and the takedown detent and spring entering from the back of the receiver. set-screw it and forget it. Some .308AR platforms have the takedown detent and spring entering from under the pistol grip. It's hit or miss by the manufacturers, but you can still capture it that way with the 4-40.
I 6-32 some of them, if they're not already tapped for a 4-40. the 6-32 is a little bigger, and easier to do, and the 6-32 set screws are easier to get at any Ace Hardware - they don't always have the 4-40s. Ream with appropriate drill bit, tap, done. If it's not already threaded, you don't have to ream the hole (standard sized hole) for the 4-40 - just tap it. The size is already there, to just cut threads with a 4-40 tap.