Quoted:
I think the RRA barrels uses the 223 wylde chambering. It is a bit longer than the 556 chambering. Check their website.

Quoted:
Applicable "codes" for 223 barrels:

Old Codes:
Three characters side by side over the chamber.
First Character:  N or W...N for NATO chamber or W for Wylde
Second Character:  8 or 9...8 for 1:8 twist and 9 for 1:9 twist.
Third Character:  A, S, or C...A for moly (chrome -moly Alloy), S for Stainless, and C for Chrome-lined alloy.

New Codes:
The actual base part number from our parts system/catalog, located over the chamber.  These are the most common.  Unless indicated otherwise, all stainless barrels are 1:8 twist and all moly barrels (chrome-lined or not)are 1:9.  All 16" moly barrels have a NATO chamber, while the 20" moly and all stainless barrels have a Wylde chamber.

200          Moly CAR
200CL      Chrome-lined CAR
202          Moly R4
204          Chrome-lined R4
205          Chrome-lined light-weight R4, 1:7
206          Stainless CAR
208          16" varmint
209          Moly 7"
210          Moly Mid
212          Chrome-lined Mid
213          Moly light-weight Mid
213CL       Chrome-lined light-weight Mid
215          Stainless Mid
218          18" varmint
219          Chrome-lined light-weight R4, 1:9
220          Moly Std (rifle)
221          Chrome-lined Std (rifle)
222          Stainless NM(rifle)
223          Stainless NM (rifle), unthreaded
225          Moly 10.5"
225CL      Chrome-lined 10.5"
226          20" varmint
228          24" varmint (1:8)
229          24" varmint (1:12)
232          Moly light-weight CAR
232CL       Chrome-lined light-weight CAR

YOU MAY START TO SEE SOME "S" AND "C" MARKINGS AFTER THE NUMBERS...THE "S" FOR STAINLESS AND THE "C" FOR CHROME-LINED...PRIMARILY TO HELP PEOPLE BOTH IN-HOUSE AND OUT TO KNOW AT A GLANCE WITHOUT HAVING TO REFER TO A CATALOG.  Additional letters are inspection marks.  All barels receive a basic inspection. and some receive an additional inspection, or even two or three...and will be marked accordingly if they have been further inspected.

Steve/RRA

Edited 07-07-06 to include chrome-lined 20" rifle bbl (AR0221).
Edited 01-04-07 to include chrome-lined lightweight R4 bbls (AR0205 and AR0219).
Edited 01-30-08 toinclude the 7"(209), 10.5" (225), and light-weight CARs and Mids (232 and 213)

Quoted:
Sorry, appears I wasn't looking close enough. The markings on my barrel are "212 C-P" followed by a small "z". So since its a 16" moly barrel I should be able to shoot 5.56 with no issues, correct?

Quoted:
I called them up last year when my upper came in unmarked.  I was assured that they are chambered for 5.56 and the only .223 they do is for the wldye chambering on select rifles.