To obtain this achievement you will need to find and collect all of the shotguns within the game. Their names and locations are:
The SP12 can be found during 3 different missions. During 'Temple City Ambush', you can find it at the first agency pickup. During 'St. Petersburg Revisited', it is being carried by some of the guards and during 'Redemption at Gontranno', your main target, Sergei Zavorotko will be carrying 1.
The Double-barreled Shotgun can only be found during the first mission 'Anathema', being carried by a guard in front of the mansion.
The Sawn-off Shotgun can only be obtained with high performance and not by physically finding it. You must achieve 2 silent assassin ranks to unlock this gun.