Here's some sample machine gun math:
I want to fill a beaten zone 10meters (~33 feet) across with bullets. I want to distribute the bullets so that they are spaced in a straight line, with around 10cm (~4 inches) between each bullet. This takes me 100 bullets. If I am firing my machinegun continuously, how will will it take to cover the entire beaten zone?
Somewhere between 6 (1000 rpm) and 15 (400 rpm) seconds of continuous fire. Even using an M-134 minigun, which fires around 3000 rounds per minute, it would take me around 2 seconds of holding the trigger down to get enough bullets in the air to do the job.
Or, I could just drop one fragmentation grenade, which would cover the whole beaten zone with it's expected kill radius, with a lot of extra area left over.
So, machineguns aren't really area of effect weapons.
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Now what about the room clearing power of the dreaded shotgun?
A reasonable rule of thumb for shotgun spreads is around 2.5cm of spread diameter per meter of distance (1 inch per 3 feet). So if you shot at someone across the street with a shotgun (around 12 meters or 13 yards for a 2 lane road) I'd expect my shotgun spread to cover a circle around 30 cm / 12 inches in diameter. Also not much of an area of effect weapon.
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And don't even get us started on authors who describe a Glock as having a manual safety...