There are 22 Weapon Locations in Call of Duty Infinite Warfare. You must scan all weapons for the Gun Nut Trophy / Achievement.

Some weapons are random drops from enemies while others have fixed locations where you can always find them. When you pick up a new weapon it will be scanned automatically. If a weapon doesn't spawn just restart the mission until it does. Replaying via mission select on different difficulty settings helps mixing it up. New weapons are glowing yellow which makes them easier to spot.

Most weapons from enemies and armories are randomized.
You can mop up all missing collectibles via mission select (Weapons Scans, Equipment Upgrades, SDF Aces & Captains). After scanning a weapon you must reach the next checkpoint, then you can quit out to the main menu.

You can check your progress at the shooting range of your spaceship (hub area, where you customize loadout before missions).

4 weapons are available from the start and cannot be scanned: NV4, EBR-800, FHR-40, Kendall 44 (that makes 26 weapons total).

The way weapon drops work is that each enemy is assigned a weapon class. Their class is always the same but their weapon changes on each playthrough (e.g. if they have SMGs they will change SMGs on each replay). Upon starting a mission it will give each enemy of that class the same weapon. There are a few exceptions to this but in a lot of missions, for example in "Operation Phoenix", you could keep killing the first two enemies over and over again. One always drops a different SMG, the other always a different assault rifle. Same when you find an enemy with a shotgun. They have a chance to drop all the other shotguns as well. A weapon that might drop in the first mission for you may not have dropped until the last mission for someone else due to this randomness.
For the video I used fixed locations wherever possible. Only 5 are random, the other 17 have fixed spawns.

TIMELINE:

#1 - Karma-45 (SMG) - 0:05
#2 - Volk (Assault) - 0:05
#3 - Kbar-32 (Assault)- 0:51
#4 - EMC (Handgun) - 1:07
#5 - R.A.W. (LMG) - 1:27
#6 - F-SpAr Torch (Heavy) - 1:45
#7 - Mauler (LMG) - 2:18
#8 - Reaver (Shotgun) - 2:18
#9 - Type-2 (Assault) - 2:57
#10 - R3K (Assault) - 3:37
#11 - DCM-8 (Shotgun) - 3:37
#12 - P-LAW (Heavy) - 4:13
#13 - HVR (SMG) - 4:43
#14 - Ballista EM3 (Heavy) - 5:12
#15 - Eraser (Heavy) - 6:09
#16 - PRP Evo (SMG)- 6:09
#17 - Spartan SA3 (Heavy) - 6:54
#18 - Banshee (Shotgun) - 7:19
#19 - KBS Longbow (Sniper) - 7:52
#20 - Oni (Handgun) - 8:16
#21 - Erad (SMG) - 8:16
#22 - Titan (LMG) - 8:16

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