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10/30/2020 2:42:12 PM
Posted: 8/26/2004 10:09:10 AM EST
[Last Edit: 8/26/2004 10:13:10 AM EST by fadedsun]

[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?

Link Posted: 8/26/2004 10:10:13 AM EST
Doh, [noob mode off].

Link Posted: 8/26/2004 10:39:57 AM EST
[Last Edit: 8/26/2004 10:44:58 AM EST by _DR]
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.
Link Posted: 8/26/2004 11:08:40 AM EST
[Last Edit: 8/26/2004 11:12:56 AM EST by fadedsun]
The upper receiver doesn't determine caliber, so technically I could build this into a 9mm AR ?

Link Posted: 8/26/2004 11:09:43 AM EST
[Last Edit: 8/26/2004 11:10:33 AM EST by fadedsun]
BTT with a meaningless smiley
Link Posted: 8/26/2004 11:32:56 AM EST
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.
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