To unlock this achievement, you must find and collect all of the shotguns within the game. There are 4 shotguns and their locations are as follows:
The SPAS-12 is carried by some of the SWAT team members during 'Asylum Aftermath' and 'Hunter and Hunted'. You can also find it in a flower box in the florist shop during 'Traditions of the Trade'.
The Sawn-off Shotgun is wielded by the bartender in both 'Deadly Cargo' and 'The Seafood Massacre'. You can also find it being carried by several bikers during 'Rendezvous in Rotterdam' as well as both of the targets during 'Beldingford Manor'.
Sawn-off Shotgun - Dual-Wielded
To obtain this weapon, you need to beat the mission 'Rendezvous in Rotterdam' with a silent assassin rating.
You can only find this gun during the mission 'Beldingford Manor' being carried by the guards outside of the house, as well as there being 1 inside the manor, near the hunting party.