I could be wrong but I thinks Its in reference to the case diameter, colt makes good kit shouldn’t be a problem.
the 5.56 and .223 difference is pressures
if in doubt call brownells
.223 and 5.56 cases are identical…the difference is in the powder charges, 5.56 being hotter and in the chambers themselves, 5.56 being a little larger to allow dirt, carbon buildup, from extended firing, Firing real 5.56 ammo in a .223 chamber, can result in some serious problems, primers being blown out of the case, cases having rims torn off leaving the rest of the case in the chamber…It’s not a good thing…
I would return your order, but not for the caliber. There are much better deals on Colt products available. You can get the entire bcg from ratworx for $40 more or even order the bolt from Specialized armament for $60 less.
Actually this is incorrect. The exterior dimensions of the brass are the same, but the thickness of the shoulder is not the same (5.56 NATO has a thicker shoulder). You are correct on the majority of difference being in the actual chamber dimensions though. This is a pretty good explanation: http://weaponsman.com/?p=604.
As to the initial question; there are no dimensional differences between the .223 Remington and 5.56 NATO AR-bolts - the difference is in the chamber and throat.
Even though Brownells is a well regarded sponsor here, $170 for a Colt bolt assembly is pretty steep. amd5007 gave some good alternative sources. You might want to consider BCM bolts for $70.
It seems to me that their prices were based on market value from years ago before Colt offered their products through other vendors or via their website. In which case, Brownells was the only retailer selling the majority of Colt items and charging a premium. I guess they never reevaluated their pricing in the current market.
In my own experience, several years ago when I was building more 1911s, Brownells was the only place to source Colt parts at a reasonable price. Now even Midway sells Colt parts.