There are but a few destructive submachine guns to be found in ME3, so make sure you don't miss any of them.
M-4 Shuriken: You'll find this gun during the Priority: Mars mission, inside the security room with Liara. Grab it before you activate the security console.
M-9 Tempest: Found inside the caves after defeating the husks and climbing the ladder during the Tuchanka: Turian Platoon mission.
M-25 Hornet: Upon taking the elevator up to the new area....
There's nothing quite as enjoyable as sniping your enemies from a distance, before they even know what hit them. Here's where to find all the sniper rifles that will let you do just that.
M-92 Mantis: Found on the ground to your right after descending the ladder during the Priority: Mars mission.
M-97 Viper: Found within the main Turian camp during the Priority: Palaven mission, resting on some boxes.
M-13 Raptor: Sitting on a bench on the level....
Here are the locations and methods for obtaining the various assault rifles found in the game.
M-8 Avenger: You'll get this during the Prologue.
M-15 Vindicator: During the Priority: Mars mission, you'll find this rifle tucked away in a small security room just off the main hallway as you approach the archive chamber.
M-96 Mattock: Found in the hallway with the red benches while doing the Grissom Academy: Investigation mission.
The location of every mighty shotgun that can be had in Mass Effect 3.
M-23 Katana: Starting weapon in the Priority: Mars mission.
M-27 Scimitar: Found within the main Turian camp, on the ground, during the Priority: Palaven mission.
M-22 Eviscerator: Found on the red couch during the Grissom Academy: Investigation mission.
M-300 Claymore: Found inside the left building within the abandoned camp during the Attican Traverse: Krogan Team....
They may not have the swagger or destructive force of some of the other weapons in the game, but pistols can be just as useful for blasting away single foes. Here's where to find them all.
M3 Predator: The first weapon you'll come across, during the prologue.
Scorpion: You'll find this on the floor after reactivating the elevator power source during the Priority: Sur'Kesh mission. Grab it before turning the elevator back on.
M-5 Phalanx: Found lying....