Hazard of Too Little Powder
It is obvious to every experienced handloader that it is important not to use too heavy a propellant charge in any given load. However many handloaders do not realize how critical it can be to use too light a propellant charge. This is a particularly critical consideration when using any slow-burning powder. Owing to heavy surface treatment, with inert (deterrent) chemicals that is necessary to slow initial combus¬tion, these powder types are harder to ignite. Generally, the "slower" the powder, the harder it is to ignite. Norma MRP and MRP2 (and similar propellant types from other vendors) fall into this category.
Should the handloader use too light a charge with such a propellant, ineffective ignition can occur. With such a small charge, much primer energy is wasted moving the charge and much primer heat are wasted heating the exposed case interior. As a result, the primer blast might only ignite a small percentage of the powder granules. The granules that are ignited can generate sufficient propellant gas to move the bullet only a few inches.
Because only a small percentage of the charge is ignited, relatively little gas is generated. Hence, as the bullet begins to move into the bore, pressure (and temperature) within the propellant plume decreases rapidly. As a result, the bullet slows and can even stop. In such a load, granules that the primer did not directly ignite do not ignite rapidly (before the buffet moves significantly), as would happen in any normal loading.
Cooling of the propellant cloud allows partially-reacted propellant gases to begin to condense onto the relatively cold, unignited propellant granules. This initiates a chain reaction: Condensation converts gas to a liquid, which dramatically reduces pressure (and temperature) inside the case, this results in even faster condensation, which results in an even more rapid decrease in temperature; once this process begins, almost instantly, chamber pressure drops to almost nothing and the bullet rapidly slows or stops.
Equally, this condensation rapidly transfers hem to the surface of the remaining granules. Since the preceding primer blast and ignition of some granules will have significantly heated those granules (often to nearly ignition temperature) and because some or ail the deterred surface may have burned away from many of the remaining granules (contrary to intuition, sufficiently rapid cooling can "put out" an ignited granule), subsequent ignition and combustion of remaining granules occurs extremely rapidly. The entire remaining charge, which is now located as a single lump directly behind the buffet, can hum before the buffet can move significantly.
As was first studied and proven by the Krupp Commission, circa 1888, whenever the ignited charge is located at one end of a relatively empty chamber (as in this instance). combustion generates a "standing wave" within the combustion chamber. This occurs because, as the charge burns, an initial pressure wave moves toward and reflects from the case head interior surface. Meanwhile, gas flow and changing pressure within the buffet end of the chamber can oppose the progress of the reflected wave. This can
Slow, stop or even reverse the direction of this pressure wave. Whenever this happens, pressure near the peak continues to increase - Las molecules continue to enter this zone (from both sides) faster than molecules leave it; all else being equal, the more molecules are in any given volume of gas the higher the pressure!
In the above situation, average chamber pressure will, no doubt, remain relatively low - often it will never reach normal peak chamber pressure; however, pressure near the peak of the "standing wave" can be several times as great as normal maximum chamber pressure! It is well proven that such a situation can wreck even the soundest gun. The resulting explosion can be lethal.
Generally, the bigger the case, the slower the propellant, and the colder the ambient temperature, the more likely this result is to occur.
CAUTION: Never use charges lighter than the suggested starting charge, as listed in a reputable, modern handloading manual.
NORMA - RELOADING MANUAL 2004 149
