these are not my words but you get the idea here...

"When the powder level goes below the flash hole, the small hole in the webbing at the base between the primer and the powder. The flash from the primer can shoot over the top of the powder. Powder is designed to burn not explode. There are 2 theories about what happens when a gun blows up from a under charge, the first is the primer lights too large of a surface area across the top of the powder causing to much powder to burn at one time causing a catistrophic failure. The second is that the flame of the primer goes over the top and starts the front of the powder charge on fire and burns backward towards the bolt face reversing the intended shock wave causing once again a catastrophic failure. I say these are theories because they have failed to be duplicated in laboratories so can not be confirmed, but the experts say they have seen guns blown up from a reduced load."

also remember if your shooting reduced load and get a squib load(bullet dont exit the barrel)and you chamber and fire another round if your lucky your going home missing a few body parts or not looking the same....stick to what the book tells you until you know a little more about hand loading for rifles.