overpressrue loads
Could any body explain the theory that by loading below min charges the pressure can rise. Hence the risk.
I have heard it said that what is required is reliable ignition of powder and the bullet to leave the barrel.
Two theories exist, both deal with a less than a full case( volume wise) of powder.
Powders such as H110, and Win296, and even BP, are influenced by the area exposed to the flame of ignition and the chamber pressure as the powder burns(deflagrates)
For these powders you will find that the loads fill the case volume from 97-110%, of available space with a bullet seated and crimped ( most loading data also instructs the reloader to use a heavy crimp on loads using these powders to get consistant results, and prevent bullets in other chambers/ the rest of the Mag from becoming loosened under heavy recoil)
When these critical powders do not fill the case completely there is an air space left above the powder when the cartridge is in a horizontal position(99.9999% of all firearms will have the cartridge in this position to fire). this allows more area of the powder to ignite at once, when the primer goes off, increasing the chamber pressure.
OR
The powder fills the case about half way or less and the flash hole is exposed, the primer goes off and does not ignite the powder instantly but drives the bullet( in a revolver here, but could be a rifle)into the cylider/ forcing cone gap. The rebounding flame then ignites the powder on what is now, esstially, an obstructed barrel.
Most reloaders have seen pictures of revolvers with missing top straps, and a cople of open cylinder chambers in reloading books due to reduced loads with some form of critical powder.
I have witnessed this first hand (though without the pysical destruction to the gun), while siteing in a 54 cal ml (for some reason the 54 is more prone to this). My buddy loaded his 54 and fired the first shot, adjusted the scope, loaded again, this time there is a huge bang and cut on his forehead from the scope, he figures that there was double the normal recoil. What we figure was happening : a ring of fouling was being created just above the powder in the barrel, and when seating the bullet he there was enough resistance to make him think that the bullet was seated properly, when it was not. Other owners of 54s we talked to had similar experiences.
hope this helps
tg