CAUTION: The following post (or a page linked to) includes or discusses loading data not covered by currently published sources of tested data for this cartridge (QuickLOAD or Gordon's Reloading Tool data is not professionally tested). USE AT YOUR OWN RISK. Neither the writer, The Firing Line, nor the staff of TFL assumes any liability for any damage or injury resulting from the use of this information.
Two events I've seen reported. One is that Dr. Lloyd Brownell wrote that he'd seen light loads more than double normal pressure in his lab and pointed out these events have a random distribution with a long tail toward the high-pressure end (something like Rayleigh distribution, I expect). The very high-pressure events happen way out in the tail, where the probability is extremely low. In other words, every time you set up the conditions for the problem, the actual event is a roll of the dice, with the particular "kaboom" combination of stars lining up being very rare (a.k.a. outliers), but not never. He published this in letters to the editor in a rebuttal to an article claiming there was no danger in it in Handloader long ago (the late '60s, IIRC).
The
Finnish Gunwriters site has a brief paragraph on a gunwriter who almost lost his sight, wearing despite shooting glasses, when a .308 Win rifle with being loaded subsonically with 3.1 grains on N320 (a pistol powder that is slightly faster than Unique), blew his rifle up, whereas 7.8 grains works just fine. One interesting point they make is that you get a good clue (and
Brownell's data indicate the same thing happening), and that is very erratic velocity, reflecting that peak pressure varies a lot, despite charge weight consistency. Also, note that the Finnish experience dismisses the idea that only slow powders present this problem. It just happens at different levels of reduced charge space. For IMR rifle powders in 30-06, Brownell showed it started occurring in the range of what works out to <60% loading density. For N320, the Finns suggest avoiding what works out to about <20% load density fill and showing good velocity consistency at that 20% level in the 308 W.
I have no proof but suspect the use of something to hold powder over the flash hole is likely to avoid the issue by keeping ignition complete. The only way a detonation would normally be possible is if the explosive mass becomes fused so a shock wave can travel through it without interruption. We see fused powder in many squib loads so that part of the puzzle has been demonstrated. What then causes the initiating shockwave is pure speculation, but it's not difficult to imagine ignitable vapors from heated, fused powder producing the equivalent of an engine knock in the chamber. But I don't know that's what happens. I just mention it to point out that nothing unheard of needs to be conjured up to propose a potential explanation.
The bottom line is that if I were experimenting with these ultra-light loads, I'd have a chronograph out to look for wide velocity variation, and I would be using polyester pillow ticking or Pufflon or something to hold powder against the flash hole to be sure it was fully exposed to the flame of the primer. I would use a magnum primer to promote that exposure. But that's just my personal thinking.
I DO NOT RECOMMEND ANYONE ELSE DO THIS.