First, there's a difference between chamber/throat dimensions and headspace.  The differences between the throats of 5.56 vs .223 is shown in the [url]www.ammo-oracle.com[/url].  This is the difference that counts.

To answer the first question: as long as the barrel is NOT chrome-lined, you can ream the chamber out to 5.56 specs.  Chrome-lining would ruin the reamer and give you a poor cut, as well as removing the chrome.

Finally, headspace (the distance between the boltface and the shoulder of the chamber) will vary between gun types.  SAAMI-spec gauges are meant for bolt rifles, and are slightly shorter than the M16 guages.  The M16 specs call for longer headspace to allow for dirty chambers and/or ammo and for slight variations in brass size.  This is necessary for correct semi and full-auto reliability.  The headspace for the M249 SAW (a belt-fed, open-bolt 5.56 machinegun) is even longer, and there are separate gauges for it.

-Troy