That means they are sized to a common set of military contractor specifications. This is to help interchangeability. SOME parts such as buffer tubes and buttstocks, just to name a couple can be aftermarket (hope close counts) or Mil-Spec. I ended up with a commercial tube that would not fit on a Mil Spec Stock. A minor pisser.
The short answer is Yes. The 6x45 is very close to the .223 and an upper set up for it should plop right on the lower and use the same buffer. In an extreme case I changed a buffer to a different weight (450 Bushmaster) but the other worked too. It is by far best to stick with complete assemblies and not go off just changing barrels etc. unless you know what you are doing. Swapping out the top or bottom half, as a unit, is pretty safe. The lower unit is the fire control, feeding, and buffering assembly.