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Originally Posted by BobbyJ
Question, when you put too much powder in the cartridge why does it not just shoot the bullet out faster vs blowing up sideways? Seems the path of least resistance is out the barrel.
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Time and momentum.
Simply put, there's simply not enough time for the bullet to get out of the way.
In open air, gun powder burns relatively slowly but under pressure, the entire charge ignites and builds to maximum pressure in fractions of a millisecond.
In high-powered rifle cartridges, the pressure goes from 0 to 55,000, even 65,000 psi, in maybe 0.2 milliseconds. In that time, the bullet only moves an inch or so. That's under normal conditions.
Gunpowder burns faster under higher pressure (generally speaking), so an over-load creates what you might see as a "run away" reaction. More powder makes more pressure, more pressure makes powder burn faster, faster burn makes higher pressure because the bullet has even less time to get out of the way.
The process is highly complex, and that description is overly simplified but it hits the main points. You can think of a gun barrel and bullet as a bottle and cork, except this cork has to travel a ways before it can "uncork" the barrel. If the pressure rises beyond what the structure of the steel can handle before that happens.... BOOM!