Receiver Considerations
The lower receiver holds the fire control group, otherwise known as the trigger, hammer and sear, as well as all the other controls for the rifle: pivot pins, magazine release, safety, bolt release and receiver extension. The upper receiver is not all that crucial and is not even considered in Sloyer’s component-by-component comparison. It should have a proper M1913 specification Picatinny rail, but that’s about it.

The receiver extension is the round tube that sticks out the back of the rifle onto which a collapsible stock is attached and inside of which is the spring and buffer that returns the bolt carrier group into battery. If this part fails, you’ve got an expensive black club.

The receiver extension—often called a buffer tube—comes in two sizes: 1.14 inches and 1.17 inches. The 1.14 size is Military Standard, but it’s really inconsequential aside from the fact that we agreed to judge by the MilSpec criteria, and 1.14 is MilSpec. The only difference is that some after-market stocks don’t fit on a MilSpec receiver extension, although most makers offer both sizes.

It’s a good idea to have a heavy buffer, known as an H-buffer, in the buffer tube. The extra weight slows unlocking of the bolt and retards the cyclic rate of full-auto guns. It is MilSpec, so it’s our basis of comparison.

MilSpec also requires the use of .154 inch diameter pins to hold the fire control group (trigger, hammer, etc.) in place. This is important if you’re going to drop in an after-market trigger from Timney, Jewel or Giselle trigger.

Now according to this compilation of MilSpec with the Sloyer’s data comparing commercial M4s component-by-component, Colt’s 6920 is the only commercial AR-style rifle that hits all of the key indicators for military standards. What does this mean? Nothing really, only that the military’s main supplier of MilSpec rifles is following all of the military standards for their civilian rifles.

For more, check out Customizing an AR Rifle, AR Operating Systems: Gas Impingement vs. Piston, AR-15 Tactical Accessories and The MilSpec Definition.