Because if you DON'T have a blank-firing device on ANY gun when firing blanks, you're doing what is basically the equivalent of sand-blasting the barrel.
Without a BFD the powders and residue don't burn fully (as there's no build up of pressures because there's no projectile to generate the pressure behind), and are shot down the barrel at high velocities, resulting in rather quick wear and tear on the rifling.
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