Fun fact: You're wrong! :D
Shotguns are great for opening doors, I agree, but by no means is it their only usage. It is completely legal to kill someone with a shotgun in a war.
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Overkill. Thats literally the only reason they are considered unlawful to use. If used to kill directly, yes they are unlawful. However, collateral damage is still feasible, so if someone happened to be standing too close to the door when you shot it, well thats not really your fault, is it.
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It was banned in the Geneva Convention, he's right