Q: You often warn readers about the danger of using reduced charges of slow-burning powders, but you don’t explain how reducing a charge can be dangerous. In other words, what actually happens to cause a blow-up?
– John McEwan
A: The problem of secondary explosion effect (SEE) has been known for a long time and there’s been a lot of theories advanced as to why slow-burning powders cause blow-ups in reduced loads.
Norma and PO Ackley decided that due to the relatively small amount of powder in the case, the primer ignited only part of the charge, which was enough to push the bullet into the forcing cone, where it comes to a halt and acts as an obstruction. When the rest of the powder ignites it forms a violent pressure wave.
Ackley also advanced the theory that the force of the primer shattered the loosely packed powder granules before they were ignited, altering the burning rate of the powder, causing it to burn more rapidly. I think this most unlikely, but who knows?
The US military placed electronic pressure measuring gauges near the base of the cartridge and the shoulder, then fired the reduced loads in a pressure barrel. Measurements were taken at both points at precisely the same instant. Comparing the results, it was found that with reduced charges of slow-burning propellants, the pressure levels were sometimes considerably different.
Back in the 1950s I experimented by loading an old Mauser .25-06 with H4831, reducing the charge from 60 to 20gn without experiencing a problem, let alone a blow-up. This agrees with the US military’s conclusion that it only happens sometimes, not every time.
But I know of a .25-06, a .240 and .257 Weatherby Magnum that were destroyed by reduced loads. It does appear that when SEE occurs, mostly in large cases with small bores, reduced charges of slow-burning powders and excessive air space.
Modern propellants are ‘progressive’ burning which means the higher the pressure the more rapidly they burn. They build their own pressure inside the case, and the burning rate continues to accelerate as the bullet moves down the bore. The differences in pressure levels led to the belief that a pressure wave was bouncing backward off the base of the stuck bullet, causing pressures to skyrocket at a critical instant.
If you want to use reduced loads it can be done safely by using a faster burning powder. Don’t tempt fate by using a slow-burning powder. Always follow the high load density requirements recommended by propellent manufacturers.
For example, the Winchester Ball Powder loading data booklet cautions against reductions in powder charges with certain cartridges and loads, warning that no reductions in powder charge or change in components should be made because such changes can cause dangerous pressures.
The people who develop loading data at Nosler hold loading densities between 80 and 111 percent, so heed their recommendations.
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