I underloaded some 4320 ionce. Almost blew up a good 30-06. I had to send it in and have the headspace reset. Underloads often explode with rifle powder as apposed to burning like it should.
I'm sure many of you know this but I'll tell you why this happens for those that don't.
When a cartridge is under loaded to point where it will damage a firearm it happens because instead of the primer igniting the powder fully from the rear where it is suppose to it sends sparks to the front of the case as well and also ingnites the front of the powder charge. When the powder burns from the front and the back at the same time it creates a huge surge in pressure when they meet.
Think of two waves in the ocean moving toward each other. When the two meet they create a single wave that is double the size of the two single waves. Much the same thing happens in underloaded cartridges.