When you enter a certain room, turn left to find a ladder leading underground. Level 3: Modified AK-47: Cuba, Battaglia Masonry. Level 2: MP38: Florida, Granados Compound. Here is a list of all the locations for the level 2 and 3 weapons to complete this and 2 other achievements (gun smuggler and modified firepower). Level 3: Vintovka SR-98: Almeida Compound. Look at the map to find the lighthouse and go there. 501 Magnum Enforcer: Battaglia Quarry Chop Shop.
When you enter the room with the safe, turn left to find a ladder leading underground. Level 3: Modified AK-47: Battaglia Masonry. On the right side of the island, inside the small warehouse. Level 3: Schofield Semi-auto: Global Storage Chop Shop. Level 3: Delta M1911 silenced: Mangano's Compound. It is located on the rooftop just to the right of the owner an engineer is required to cut the fence. Level 2: Spitzer Centerfire: Emilio's packing company.
Found during the mission where he sends you to rescue one of his soldiers. Level 2: Silenced Pistol: Ryan Roth mission. 44 Magnum Force: When attacking Carmine Rosato's compound in New York, go upstairs in the tiny foyer before you approach Carmine. After he torches it you will find it right behind it. From the inside of the compound looking at the front gate, you will see a hedge on the right you can burn with an arsonist. There are a total of two (2) in New York, five (5) in Florida, and three (3) in Cuba. They are now organized by city, listed in the most likely order in which you will come across them in that city. I've taken the list that TheSpiderHam666 provided for these Achievements, and rearranged all of the weapons. Here is a list of all the locations for the level 2 and 3 weapons to complete the 'Welcome to the Gun Show', 'Gun Smuggler', and 'Modified Firepower' Achievements.