Florida shooting: Why the NRA wields so much power

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Media captionPresident Trump has become an enthusiastic supporter of the NRA

Every time there's a mass shooting in America there are calls for action to stop it from happening again. But any effort to introduce stricter gun laws always falters in the US Congress - and that's in large part because of the power of the National Rifle Association. Here's a closer look at how they wield that power.