A gun that's capable of slam-firing tends to wear down the mechanism to a degree that if done enough, it will fire whenever you bring the pump forward (which is BAD).
Not to mention Mossberg 500s, Remington 870s, and lots of other shotguns have triggers that don't allow slam fire. Why does it matter that this one doesn't too?
If it's so desirable, just modify it or hire a gunsmith to.
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