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55k CUPS for .223 SAAMI. 62k CUPS for 5.56. LOA spec for both .223 and 5.56 are both 2.260 in every reloading manual I've seen. As noted, the difference is in bullet to land/grove engagment as the primary difference.

Got my info from Here.

So yes, a stainless bull barreled match grade .223 that could probably handle even the SAAMI safety net of 78k CUPS can handle the extra pressure from 5.56.

Now, since an AR newbie probably isn't going to be all that interested in dotting every "i" and crossing every "t" on their first purchase, a simple warning about the very real differences in pressure seemed warranted without going into 11 pages of techno-babble.

Capice?

Absolutely understand your point about barrels and the differences between them. As long as the pressure vessel (chamber) portion in your bull barrel is larger than those with a “pencil” barrel your comparison may work well. As many bull barrels simply carry the chamber portion forward to the end of the barrel this comparison doesn't always hold true. Additionally, even the “pencil” barrel is only thin well past the chamber of the firearm, where pressures rapidly decrease.

You also have to consider the other parts in the equation, bolt strength, lug strength, and gas pressure in gas powered actions (AR 15 for example). Exceeding the recommended pressure in the barrel also causes increased stress on all of these components. Do it often enough and you have potential for breakage of supporting components like those mentioned.

As previously mentioned, the concern using 5.56 in a .223 chamber has nothing to do with the barrel profile, rather jamming the 5.56 in a tight .223 chamber basically means that you are shooting a proof load every time you pull the trigger. Not a good situation.

Thanks for the discussion.