Mistakes that destroy firearms and perhaps harm the shooter are a nightmare scenario for handloaders. They do happen, but they are probably less common than many believe. Sadly, when they do occur, it is almost always the case that the incident could have been avoided by following a few simple rules. The place to look for safer solutions begins with understanding propellant burn rates and how pressure is created within a cartridge. 

In a rifle cartridge, propellants can be viewed broadly in three distinct categories. There are powders that are too slow to bring the cartridge to peak pressure , no matter how much propellant is added to the case. A handloader electing to load their 308 Winchester with Hodgdon H50BMG cannot get enough propellant into the cartridge to begin to achieve dangerous pressures. The next group is propellants that operate within a burn rate appropriate to the cartridge being loaded. These are the loads you find in manuals. The last group of propellants, the ones that account for most high-pressure events, are powders that are much too fast for use in the cartridge. Universally, the fault in this instance lies with the handloader. 

Mixed Powder in a Hopper Destroys a Cooper Model 21

One of my jobs when I was with Cooper Firearms of Montana was to manage customer service and the Shooter/Testers, whose job it was to reload for and test firearms.  One of the high-pressure failures we received from a customer involved a Model 21 in 17 Remington. The rifle had held together but only because the locking lugs had obturated into the raceway locking up the bolt. The receiver had bulged but not shattered. When we finally got the barrel off and the action cut open, you could see the headstamp of what was left of the Remington case imprinted on the bolt face. The interior of the rifle was coated in atomized brass. It looked like the action had been plated in gold.  

When I talked with the owner, he was shaky about the incident and was honestly more interested in why it had happened than in trying to shift the blame to us as the rifle manufacturer. The answer lay in his powder thrower. He had been reloading pistol cartridges and later in the day switched to his 17 Remington. At my request, he dismantled his other 17-caliber handloads from that session and found one that had a mixed charge of pistol powder mixed with the extruded rifle powder intended for his Cooper. Because there had been pistol propellant left in his powder hopper when he switched to rifle propellant, the first several loaded rifle cartridges received a dose of the wrong powder, and a valuable rifle was destroyed. Best practice here is to always have just one powder on your workbench at a time, and always make sure your hopper is emptied at the end of each reloading session.