Please tell me you are an invisible six foot rabbit.
No rabbit here, just an old fart with over sixty years behind the trigger. But what part of "in a safe, distant and controlled environment" is not clear?
The OP isn't entirely descriptive as to the detail and fallout of CF event. Photos and measurements of the chamber, I suspect, will be forthcoming. But if you ask me right now, there are a number of things that could cause what has been described. In reverse order of potential:
Barrel obstruction - Highly unlikely, as warranted by OP.
Ammo - Unlikely, as OP warrants reputable ammo, and the failure to the brass suggested brass was un-contained.
Human Error - Remotely possible. Bolt not rotated to fully engage lugs, but enough to set firing pin.
Bolt face issue - Possible, but cartridge rim must have been captured by the extractor for ignition.
Head space gap due to sizing for caliber other than .308 WIN - Probable, given rupture above cartridge rim, and ubiquitous ".308" caliber stamp on barrel. Shame on builder's smith.
Head space gap due to faulty installation of barrel - Probable, due to rupture above rim. Chamber OK, but barrel not fully threaded into receiver. Shame on builder's quality control.
The OP mentioned that he did not feel the tangible final "resistance" when closing the bolt. This ever-so-slight resistance is not normally due to the case seating fully into the chamber, but more so the resistance of fully engaging lugs and of the firing pin spring compressing into lock-up. So I don't give this a whole lot of weight.
In any event, once the old brass is cleared from chamber, and no obvious chamber or head spacing measurement errors are evident, forensics will demand that the rifle be fired again under safe and close supervision of OP's third party for the record. As suggested, this will eliminate a number of the possible CF causes listed above.
Don't know about you, but I'd like to watch that ... from a distance.
