Well, probably not a bad idea to fact check me but if memory serves, it goes something like this. If you don't have good full ignition, the smaller explosion pushes the bullet into the lands and builds pressure and a delay which gives a secondary explosion that is usually way over pressure. Also , low case fill when shooting at a downhill target can allow a gap between the primer and powder causing same effect. Ball powders being more susceptible to this. You shouldn't try light loads in weatherby cartridges or any cartridge with a lot of freebore and also should be very cautious if experimenting with faster than normal powders while working up a velocity ladder with hammer bullets. I could be slightly off on this but it's been about 20 years since I've actually read a decent reloading book but I'd guess a google search would probably explain it better and may more accurate. Hope that helps
