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I am talking about, for example, debris in a gun barrel causing the barrel to explode. I would think that if the debris were less resistant than the metal of the barrel, then the expanding gases would tend to force the debris out of the barrel. But at least in movies and what I think I have read, debris can have a much bigger effect and in fact, can cause the gun to explode.

If this is true, what causes this effect? Again, why is enough force generated to damage the gun rather than dislodging material like, I don't know, soot or the remnants of gunpowder after it burns?

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The answer is that the debris has to be accelerated in addition to any resistance from being lodged in. So if it is lodged in there where it takes you one pound of force to push it out with your finger, the “acceleration resistance” due to accelerating that debris is negligible for your finger. But in an explosion the debris must accelerate so fast to keep up with the expanding gas that the acceleration of the debris becomes meaningful and requires force to make it go from speed of zero to let’s say 100mph in the blink of an eye. If we assume mass of debris m, and it must be out of the barrel in time t (a low time, like 0.001 seconds) before the barrel would explode, over a barrel length distance of d, then the force is f=ma where a is the acceleration, a = 2d / t^2 (from d=0.5 a t^2) and remember t is low so t^2 very low.

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  • $\begingroup$ so you are saying that indeed relatively light-weight material in a gun barrel can cause the metal of the barrel to bend? this is not a myth, then? $\endgroup$
    – releseabe
    Commented Jul 19, 2021 at 10:59
  • $\begingroup$ Right. It depends also on whether it blocks the explosion from just going around the sides of it, which I didnt mention. I also didnt mention that introducing asymmetries to the explosion could bend it. So if it blocks the explosion but not evenly. (A bullet is symmetrical). But the key idea is that you have to accelerate the blockage uber fast and can generate some force that way too, and if also not symmetrically then... If it is light ENOUGH then no it couldnt. I cant say they aren’t exaggerating at all, but it’s perfectly plausible and can happen $\endgroup$
    – Al Brown
    Commented Jul 19, 2021 at 11:10
  • $\begingroup$ That symmetry thing is important. Round stuff is very strong when pushed on evenly without local stress points $\endgroup$
    – Al Brown
    Commented Jul 19, 2021 at 11:13

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