I am not ready to give up on the risk of detonation in pistol cartridges - have not experienced it but have discussed it with folks well versed in interior ballistics. Here is an article that I happened on - a resonable statement of my thoughts on the matter:
Shooting Sports - Light load detonations
M. D. Smith did an excellent experiment using light loads and repositioning the powder in the case to demonstrate the effect of uneven burns and pressure waves on exterior ballistics. In short, powder positioned by the primer produced consistant velocities while powder positioned by the bullet produced very erratic velocities. These results are consistant with current Interior Ballistics thought.
The interior of a cartridge in combustion is not an ideal gas at rest. There is a flame front, pressure waves, and two densities of content present (air and powder/air). As you know, sound waves propagate through different densities at different speeds. It is a complex environment to model, and no single physical rule is sufficient.
Generally speaking, an air gap in a cartridge causes the flame front to extend quickly over the exposed surface of the propellant. Because of gravity, this air gap (and flame front) tends to be along the top of the powder, because the powder has settled along the bottom.
Some powders work well in this situtation, like a little Bullseye in a big .45 Colt case. It has lots of room to expand and quickly reaches peak pressure and complete combustion, resulting in an economical and efficient load. Bullseye is a double-base powder with lots of energy content and little or no retardant. It needs little more than a spark to ignite it.
Slow single-base powders or heavily retarded double base ball powders need pressure, not just ignition heat, to burn well. They generally burn well in a full case with no air space (other than inter-powder space). In cold conditions, with weak primers, and/or low pressures (light loads) it is quite possible to get a "bad burn" (large percentage of propellant never ignites.) This manifests itself in erratic velocites, hangfires and so-called "bloopers".
Occasional detonations of light loads of slow powders was well debated in the 50's and 60's and is now an accepted fact of interior ballistics. They are easier to understand given the facts above, and the fact that most energetic materials have two burn rates, a deflagration rate and a detonation rate. Propellants are distinguished from explosives (by the knowledgable, anyway) as energetic material being consumed at its deflagration rate. Explosives are self-consuming at their detonation rate.
Most energetic materials can be consumed at either rate, given certain circumstances. Dynamite lit with a match will produce a blazing bonfire, but no explosion. Expose it to the concussion of a blasting cap and it will detonate. Light it with a match in an small, unvented closed chamber (called a "bomb") and it will transition from deflagration to detonation once the pressures get high enough (assuming it is not already completely consumed.)
Detonations of small charges of slow powder work the same way. They deflagrate (relatively) slowly. Often pressure waves move the powder around and pack it against the base of the bullet. Single base extruded grains often break and are crushed by the slowly rising pressure. Now propellant is burning front-to-back, where it was burning top-to-bottom and/or back-to-front.
Pressure waves then bounce back and forth along the case, sometimes meeting each other in opposing directions and producing peak pressures nearly twice the expected, given the volume of gas. This impact or concussion sometimes pushes the burn rate of the propellant into its detonation rate.
Modern reloading manuals warn against reducing loads of W296 and H110 in magnum pistol, and H4350/IRM4350 and slower in magnum rifle, for this reason.
The .308 and .30'06 are relatively immune to this phenonenon for two reasons. One, the exposed surface of the base of the bullet is fairly large compared to the case volume. Given the same crimp and debulleting force required, debulleting occurs sooner than in a .308" than a .277" because at "X" PSI there is more square inches (or fractions thereof) of bullet base for the gas to push against. Debulleting considerably moderates unfavourable pressure wave situations by increasing chamber volume (lowering pressure, giving pressure waves longer travel, etc). Chamber volume increases quicker in a .308" bore than a .277" bore for each unit of bullet travel because of the four-fold increase in bore volume for each corresponding increase in bore radius.
The second reason is proximity. The cases are small enough that either primer spark, or the primary flame front will reach all the powder relatively soon in the sequence of ignition. When a signficant amount of powder packs-up in the front of the case, unignited, the chances of detonation greatly increase.
Understanding all this aids the reloader in choosing the proper burn rate for each cartridge and load, though some experimentation is always worthwhile.
Ken.
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