First, the photo:



From left to right: Rock River, Smith & Wesson, Bushmaster, DPMS (with Rock River Upper), and Colt.

Second, the question:

All of them have "5.56 mm" on the lower receiver EXCEPT the Colt, which says ".223".  However, the Colt barrel says "5.56 mm".

So, can I (safely) fire 5.56 mm out of the Colt? I know that 5.56 mm out of a .223 receiver is generally advised against, but I thought all Colts were capable of firing 5.56 mm. I didn't know the lower would only say ".223" until after I received it. (Pardon the lack of AR fu, but I'm still an AR newby, just collecting a few things while I can.)

I love family pics.  I'm guessing you were/or are a Squid Supply Ossifer.  

Quoted: I love family pics.  I'm guessing you were/or are a Squid Supply Ossifer.

Seabees Can Do!

We Build, We Fight!

(Guys, I just realized that perhaps I should have put this in the AR Forum, so pardon the single AR question in Georgia HTF.)

Barrel markings matter not lower markings. You can shoot .223 out of a 5.56 but you cannot shoot 5.56 out of a .223 Remington barrel. Consider .223 a .38 spl and 5.56 the hot .357 mag of the mouse guns.

Checked the barrels:

S&W - 5.56 mm    1/9
Bushmaster - 5.56 mm     1/9
Colt - 5.56 mm     1/9
Rock River - (blank - will have to research)

Thanks for the info guys.

RR may be under your handguards...

Quoted: RR may be under your handguards...

I just went to see if this was the case, and no such luck. I saw some numbers, but they look like production numbers or something, as opposed to caliber and barrel twists.

I found some help:

ar15.com/forums/topic.html?b=2&f=38&t=149779

Looks like I have NATO chambers based on the numbers 200 and N9A.

It took some searching, but I found it.

Thanks for the help.