Well, I wasn't going to post this til next week--but had a change of heart cause I thought I should for safety reasons. However, I won't jump to any conclusions until i gather more information and try to get some input from industry experts.
Today I kaboomed my 350 L build.
As far as i can tell, i'm the first to achieve this dubious distinction with the 350 L.
The basics are this: I retested the same loads I shot yesterday because conditions were warmer and not nearly as windy. The first load was 28.9 gr of 1680 driving the 180 hot core sized to .357. The same load yesterday was getting me an average velocity around 2,065 fps +/- COL was 2.27. The one thing I did different today was use the starline brass untrimmed at around 1.704 as opposed to what I formerly trimmed to 1.701. While that is the only thing I changed, I don't see how this can account for a catastrophic failure. During loading I doubled checked my RCBS feeder against my balance--so I know that was OK. The dummy rounds made to the specs drop in and out of the chamber freely, and when dropping the bolt on a cartridge it comes right back out with no resistance--nor is there any evidence of the bullet engaging the throat or lands.
Here are the pictures of the failed case which ruptured on the side. Looking at the SAAMI specs--the area of expansion seems to match (.2") the depth of the area from the rim face to top of web. One key thing I never noticed before when looking at the SAAMI chamber specs--it appears that the headspace datum is actually the bolt breech face (in other words the case does not headspace off the mouth, at least as far as I can tell). I'm not sure how that works in the world of AR's, having never encountered that before.
The bullet exited only slightly faster than they did yesterday--around 2100 fps. the magazine was destroyed (sorry about that mehavy) the bolt extractor was (naturally) destroyed as well. The charging handle--which was old and well used/abused, failed the first time I mortared the gun. The bolt was seized in the upper due to the bolt lugs being locked to the case in the chamber and the carrier having back out.
Having kaboomed an AR before, I knew that if I could gradually work the lugs so they moved--and managed to turn the cam pin in the upper receiver slot enough--I could get the bolt and carrier loose eventually. It took me a couple of hours, but it worked. I took the entire upper apart inspected cleaned etc and didn't find any notable damage other than what I already mentioned. I do not believe this was an exceedingly high pressure failure as you might find in a powder pressure spike (which I've done) because there was no evidence of brass vaporizing, it seems to have simply shredded on the side of the head. Within two hours I completely repaired and reassembled everything, and went out and fired some factory hornady American whitetail. No issues. Also, as I had the barrel and components apart, I also replaced the Rex Silentium comp with a JP brake--the reduction of the concussive report sent back to the shooter was noticeable--at the price of only a slight increase in felt recoil--which now feels about the same as that of a 5.56 carbine. So what.
There's the basics.