In the early years after 9/11 the suicide belt, the car bomb and the homemade explosive device were the weapons of choice for jihadis: hidden, brutal and hard to counter. But as 2015 heaves to a close, its atrocities littered across the calendar – Charlie Hebdo, Sousse, Garissa, Tunis, Copenhagen and Paris – it is the AK-47 that has come to the fore. Across Europe more terrorist attacks have been...