The Firearm thread: Engineering artistry of wood, polymer and steel.

[August]

Originally Posted by Ducimus

Well, if you get a 5.56 NATO chambered upper, its really all you need. Headspace aside (haven't done my homework on .223 and 556 headspacing, I'm assuming their same), you have the chamber pressure rating to feed it 5.56 or .223 at that point. My wife can feed either one through her rifle without any issues whatsoever.

EDIT: I think the only major difference is more brass expansion with .223, the cases are slightly thinner then 5.56, and NATO chambers tend to be a bit looser to allow for dirt, and debris.

Correct. The idea is to be able to swap between them quickly. Good idea on the flat top rail too.