blowback vs fixed breech
welp, in a blowback gun, as the primer is fired and the powder burns, the brass usually begins moving rearward a bit. then as the bullet leaves the barrel, the pressure in the barrel "blows back" to slam the casing further rearward to rack the slide.
if the casing is bulged or weak, it can crack, allowing too much pressure to escape while the bullet is still in the barrel. if too much pressure is allowed to escape before the bullet exits the barrel (in other words, the pressure can no longer propel the bullet forward) and if the blowback is not powerful enough to move the casing further back, the pressure builds to a high enough level in the barrel to cause rupture. (maybe this can also happen if a crappy primer pops out after being fired)
the hole or tear in the brass is usually not large enough to allow all the pressure that is needed to propel the bullet forward and the casing backwards. if in some case a really large tear formed in the brass and was somehow able to travel rearward enough to cycle the slide and another round, then you may end up with a bullet still lodged in the barrel. upon the next round being fired, a barrel rupture is expected.
also, if the casing bulges enough, in an extreme example, it even hits the top of the clip, not allowing the slide to cycle properly.
*whew* and i only have a basic knowledge of how this all works, i am sure there is some guy out there who can expalin blowback and everything in a really scientific way. i think i remember reading that as the bullet exits the barrel, it creates a vacuuum therein, and then the implosion (not explosion) is actually what cycles the slide. maybe i got that part wrong, but it seems like that's what i read.
can anyone explain this better?