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There's a lot to consider when looking at the strength of the AR platform. I personally do not consider them overbuilt by any stretch. However the fact the large bores have been in use for awhile tends to show it will be ok.
Breech strength or "integrity" of a rifle depends on a few different factors. One is the overall peak chamber pressure of the round and directly related to that is the amount of that pressure that gets translated into force on the bolt head. The larger cases will transmit more force to the bolt because of the increased surface area however the large bore round may operate at lower chamber pressures.
How this force is handled depends largely on two elements, the amount of area of the lugs that is in contact with the lug abutments in the barrel extension, and the amount of area that attaches each lug to the bolt body (called shear area) along with the shape of the lug (length vs width vs height)
AR's are notoriously weak in the bolt area, in almost every .gov/.mil endurance test I've read on the M16/M4 at least one lug shears off if not two (those being closes to the extractor. This is with 5.56 ammo. Over the years various blends of steel have been tried to combat this problem (one mitigating factor is the bolt getting heated up so much)
Additionally AR uppers are not super strong for hanging off a long heavy barrel and this is why you see the beefed up aftermarket uppers (and things like Mark Larue's OBR barrel attachement).
If you look at the path of the recoil forces in an AR, it seems almost surprising it holds up. The force is transferred through the bolt to the barrel extension to the upper receiver in an area that is not particularly thick, is threaded, and has a notch cut in it creating a stress riser. From there is is transferred to the lower through either one or both takedown pins, or by direct contact with the buffer tube tower which also doesn't seem very strong to me, then through a threaded joint to the buffer tube, then to (in some cases) a single steel pin in a hole in the aluminum tube.
The kinetic energy of the bolt carrier assembly can be controlled to a large degree to be similar whether in an 5.56 or 458 SOCOM.
Justin
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