My best guess would be because the chambers of the machine gun (I'm assuming we're talking about a belt-fed machine gun here) are not properly cool or are being over used (much like in any firearm.) The consecutive blasts in the chamber would heat it up. The time between the rounds being prepped and fired would give it some time, but not enough time, to cool down, thus the temperature would slowly rise until it reached above the flashing point of gun powder, and the heat would dissipate into the cartridge itself, flash the powder, and send it on its way
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I be the anti-myth rythm rock shocker
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