. The ultimate danger in subsonic loading is a phenomenon known as Secondary Explosive Effect (SEE). SEE can destroy the strongest rifle action and it can happen on the first shot or the tenth. In fact it is more likely to happen after the bore is dirty from the first few shots. SEE is the result of slow or incomplete ignition of small amounts of smokeless powder. The powder smolders and releases explosive gases which, when finally ignited, detonate in a high order explosion. The warning sign for imminent SEE is a hangfire or delayed ignition.There are a number of techniques that can be used to prevent or delay the onset of SEE. These involve specially prepared cases as well as bullets, in addition to selecting a suitable powder and primer.
Fluffy (low bulk density) powders like Red Dot take up much more room in the case. Load density is safety in a subsonic load. It is suggested that loads never go below 30 percent load density (powder occupies 30 percent of the case volume) unless you want to dance with the devil and SEE
Load density can be measured by dropping a length of wood dowel into the loaded case. Mark the dowel where it meets the case mouth. Then pull the dowel out and compare it against the case. Keep notes on load densities for every powder and case combination so that safe loads can be identified at a glance before they are loaded. Don't go below 30% load density!
Note this is copy and pasted off a post on snipers hide by Top Predator... His explanation is far better than mine
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