if i had a dollar for every time someone has asked me, "will it explode when you shoot it," i'd be able to afford to buy a "real" one.
no. it won't go "kaboom," because the lower really doesn't handle the pressures of cartridge ignition. it's that simple.
such pressure are contained in the barrel extension and the bolt. and, apparently if something does go wrong, the upper takes the pounding, and the lower emerges pretty much unscathed.
photographic evidence. ---
caption: you'll notice that the threads to the upper appear to have remained intact, and still under the barrel nut and the retainer for the hand guards. the barrel extension is on the end of the barrel, and the bolt appears to have remained in battery when the cartridge went off. the upper receiver is split in half lengthwise, from top to bottom, but, the forward lug and receiver threads are still attached to the lower by the assembly lug.
now, the fascinating part. the bolt carrier has been blown in half, as well, and it parted company with the bolt which remains in battery, with the firing pin and the cam pin still in it. i just couldn't hazard a guess how that happened, or what happened to the gun to cause it to go up like that.
sobering, isn't it. how'd you like to have your head next to the thing when that happened?
but, for our discussion of the "plywood" lower. look at the lower receiver in the above picture. it appears undamaged, or if it is damaged, that is not readily seen in the picture. the magazine lips to the magazine are a bit mangled, but, the receiver itself just wasn't exposed to the pressures involved to have been damaged in the least by them.
the above picture is found at the following link, but, it is unattributed.
i hope this helps people understand and accept my statement that the lower receiver in an ar-15 does not bear the forces of cartridge ignition, and, as in this case, can be really close to when something lets loose else and escaped unscathed from it all.
john jay @ 01.31.2013