Nighthawk has referenced P. T. Kekkonen, who has now passed on, a couple of times. P.T. did give a little more detail on what happened than what has been reported so far.
A main reason of S.E.E. is disorder of powder ignition. Powder charge does not burn after the explosion of a priming pellet. It smoulders like a German tinder, developing a cocktail of explosive gasses like nitrogen oxides, hydrogen (very reactive "In Statu Nascendi" hydrogen - not yet bound to H2 molecules), and carbon monoxide. When this highly flammable mixture of gasses catches fire from still smouldering solid powder remnants, may the "BANG !" be horrible. Mere three grains of gasses may literally wreck the strong .308 Win. rifle action. (Three grains of smouldered solid powder is still three grains of material, despite of it's gaseous form of existence).
Some of the details of the incidents are:
Highest measured detonation pressure was 10 000 atmospheres. A pietzo-electric pressure gauge was broken and highest grade on the pressure scale was this 10 kilobars. A sturdy test-barrel of a German gun-proofing laboratory was wrecked, of course.
This disastrous test was repeated with another set of equipment for the sake of comparison. Pressures of first shots were slightly less than normal. It might be fifth or sixth shot, when the new test-shooting barrel blew up. Again a pressure gauge disintegrated and a scale told: 10 000 atmospheres! It was presumably just a fraction from whole horrible truth, because so called "wave pressure" of a detonation may exceed reading A HUNDRED THOUSAND ATMOSPHERES, when the explosive material is in gaseous form of existence, pre-heated and pressurized before explosion.
Caliber of tested cartridge was .243 Winchester, bullet weight 80 grains, powder then-new NORMA MRP, and the charge... surprisingly... just 15 % less than a maximum (compressed !) load. It was STILL A REDUCED CHARGE DETONATION; not one caused by an excessive charge, because the charge could not be excessive with those components in use. Light bullet and slowly burning powder is not an advisable combination of loading components for .243 Win., known as a caliber prone to S.E. Effect. (It's "big brother" .308 and "kid brother" .22-250 are considerably less risky; last mentioned presumably because of more steep 25 degrees shoulder angle).
Needless to say: All the loading components were examined carefully afterwards. They were faultless. Just the burning rate of powder was selected wrongly for the bullet weight. MRP powder is O.K. for .243 Win., but for the heaviest bullets of this caliber; weight 100 or 105 grains. For the most usual 90 grainer bullets is some more fast-burning propellant advisable.
Noted was a slightly less than a tenth of second lasting delay between hit of a striker and explosion. This same delay is noted also by survivors of S.E.E. accidents, if they can remember something from the "big bang". (Usual recollection is: "I squeezed the trigger and woke up in the hospital"). If the delay lasts a second or more, it is just an usual hang-fire, without signs of excessive pressure.
The warning is the same as what Ken H. has said in regards to using loads of at least 90%.
I have spent a few years tracking the incident down, from P.T.'s reference to
P.S. That story about two broken .243 Win. test-barrels and purposeful courting of S.E.E. in Germany is told in the book "Handbuch f�r den Wiederlader" by K.D.MEYER, who was then a director of German DEVA Institute.
One of the Swiss members of the S&W forum has a copy of the referenced book and provided this translation from German.
For getting a starting-load, orient yourself on rounds with known load-data. Look for rounds with similar case volume and similar bullet diameter and reduce the max.-load of this rounds 25% (better 30%) Don't do this with very progressive powders like f. e. Norma 205, because "underloads", like described anywhere else in this book with this kind of powders can cause S.E.E.
This effect is still a problem for the ballisticans. In the proofhouse of the city Ulm, happened this effect two times in a very short time period. The pressure-measure was dimensioned very big so this part could be used further after the S.E.E. But most of the other parts and the test barrels were destroyed. If fired in a hunting rifle, this load would destroy the rifle in numerous parts.
My information came from one of the 18 parts from
http://guns.connect.fi/gow/QA.html
