For those who've not seen this before, the treatise on using small charges for double-based pistol powders in rifle cartridges in 'Levergun Performance Studies' is well worth reading. It gives a very convincing explanation of how disc shape pistol powders ignite and burn in very low fill-ratio applications, and why the practice is very safe (provided as always that overloads aren't incurred).
http://www.gmdr.com/lever/lowveldata.htm
Detonation from too-low charges is linked exclusively to very low fill-ratios of coarse grained slow burning powders in large capacity / over-bore capacity rifle cartridges, primarily magnum types. That is why many larger magnums have relatively high starting loads listed for their 'standard' propellant grades and a warning not to drop below them is given. Even there, it has proven very difficult indeed to replicate detonations. When Hodgdon Powder first received occasional reports of these occurrences in the Weatherby Magnums, and a few other large cased numbers way back in the 1950s or 60s, they were initially sceptical. When eventually convinced, they loaded and fired hundreds of such likely dangerous combinations and failed to get a single over-pressure result never mind a rifle destroying detonation.
That's not to say that excessive under-charges are ever a good thing. Even in smaller cartridges such as 308 size, a too low charge of a 'standard' burning rate powder can produce mild hangfires with consequently large ES values and poor performance. Moving to a faster than normal burning powder normally cures this. Hodgdon still lists 'youth loads' of H4895 in several popular deerstalking cartridges, some loads getting on for 40% below normal. Speer used to publish very low velocity / low fill-ratio reduced loads for jacketed bullets in a range of cartridges in its manuals, usually employing IMR-4198. These would give MVs at around half normal levels, but it was strongly advised not to reduce minimum listed loads - not for the risk of detonation, but rather that jacketed bullets need a minimum pressure / speed to behave consistently in the rifle barrel, and of course if loads are reduced too much, there is a serious risk of leaving a bullet lodged within the barrel. Lead bullets give much greater latitudes as they work consistently at significantly lower pressures.
