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Facebook has recently banned people sharing blueprints for 3D-printed guns and now a group of US Democratic senators is urging other big tech companies to follow suit. But why would someone choose to print their own weapon, instead of purchasing a traditional firearm? Cassandra Crifasi, from the Johns Hopkins Center for Gun Policy and Research, explains - and discusses the battle to stop their proliferation.

(Image: A 3D-printed gun - credit: Kelly West/AFP/Getty Images)