My AR is marked .223 - 5.56 so I purchased .223 reloading dies. I've been shooting ammo head stamped 5.56x45 PPU07, and FIOCCHI and Winchester .223. I assume the PPU07 is Mil Surp ammo. When I run the 5.56 into the .223 dies I'm smearing the surface of the brass near the base. This doesn't happen when I run .223 brass through the dies. I don't know if the Mil brass is harder or softer, but it looks like I won't be reloading the stuff.
Update - 2/9/08
My AR is custom assembled with a DPMS lower marked .223-5.56, and a barrel marked 5.56.
I also have some Lake City 5.56 brass that works in my .223 dies. The Lake City 5.56, and FIOCCHI and Winchester .223 resize in the .223 dies just fine, but the 5.56 PPU07 doesn't.
My Hornady & Sierra reloading books show no difference in cartridge dimensions, and niether does my Manual of Cartridge Conversions, though it seems to be common knowledge that there is a slight difference. Looks like a little research is in order, maybe SAAMI will tell me something.
Update - 2/10/08
Well SAAMI didn't help any. They want payment for just about any little bit of information, and I'm not in the paying mood. If this is correct, the biggest difference is that the 5.56 is rated for a higher pressure and the distance to the lands is a little longer than the .223 chambering. Assuming the nominal dimensions are the same, I'd bet the tolerances are different.
Apparently nobody reloads to the 5.56 specifications, because I didn't find any dies for it at RCBS or Hornady. Dies for the .223 are available though.
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They're not the same thing, contrary to popular belief. I assume you've got a Bushmaster, since they're the one manufacturer I can think of that marks receivers like that. I wouldn't count on it being a 5.56 chamber unless you get somebody with a Christensen chamber reamer to check it and be sure its cut that way. 5.56 brass is, IIRC, thicker and the cartridge has a longer leade than .223 does.
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