Hola.

[Noob mode on] I'm building an AR15 rifle intending to be chambered for the 5.56 (have loads of it around, XM193 and a few Q3131A) since I plan to shoot mostly military surplus. An upper can be used for either (5.56 or .223 build )  correct?

Ben

Doh, [noob mode off].

Ben

The upper receiver does not determine the chamber type.

concerning chamber type:


there are two schools of thought on this.

1) if you have a 5.56 chamber you can shoot both .223 and 5.56 cartridges with no problems; If you have a .223 chamber you should shoot only .223 cartridges as chamber pressure can get very high when shooting 5.56mm cartridges in a .223 chamber.

2) it does not matter, you can shoot either cartridge in both chambers.


I personally believe that it is imprudent to shoot 5.56 in a .223 chamber, especially hot military loads like LC XM193, as the leade in the .223 chamber is shorter, creating a tighter fit, which could, in theory result in pressures which could lead to case separation or other issues.


HOWEVER...I am no expert on the matter, I am just going by what I have read. I tend to err on the side of caution in these things, and not ignore the SAAMI warnings.  

Armalite, Inc., for instance, takes the position that it does not matter.

I am quite happy to buy all my rifles in 5.56 chambers and forego the minute gain in accuracy that a .223 chamber may impart.


But you will have to make up your own mind. This is one of the issues that tends to be hotly debated here.

The upper receiver doesn't determine caliber, so technically I could build this into a 9mm AR ?

Ben

BTT with a meaningless smiley

Yes, actually you could.  There are several sources out there for this.

It's more accurate to say the upper -itself- doesn't determine caliber.  I've seen AR lowers shooting 50BMG.