It's got to do with pressure.
Firearms all work within tolerances, exceed them and things go bang, in a bad way.
As the gasses push the projectile along the barrel great amounts of pressure are created. The projectile doesn't run freely along the barrel, it touches it all the way along and great amounts of pressure are required to push it through. T
he muzzle (hole at the end of the barrel) is the only way for the pressure to escape and it all comes out after the projectile, along with the muzzle flash (fire) left over from the explosion that forced the projectile out.
If the pressure is too great (called over-pressure) it can find weaknesses in the barrel or firearm mechanism and exit that way. Having a rifle bolt explode back into your face will take it off, along with half the head, and the pressure if great enough can blow out the barrel which can remove fingers and hands...or the entire rifle may grenade itself as below.
This is the simple version but I think you get the idea.
Here is an image of what can happen with ammunition over-pressure. (Image from https://www.primalrights.com/)
wow! didnt know that it could have such devastating consequences!
Yep, people die.
I'd not have wanted to be the one behind that long gun in the image.
Firearms are not toys and in the wrong hands can be devastating; I don't just mean by shootings and all, but stuff like over-pressure. I'm fortunate enough to know a few things after about 33 years of shooting so understand the dangers and how to mitigate them. Many do not...and video games and movies don't teach them.