There are 14 possible weapons you can use whilst on the mission Old Comrades, and they are listed below:
PistolX18
.50 GS
1911
M19
SMGMP7
UZI
P90
ShotgunModel 680
725
Assault RifleFR 5.6
FAL
AK-47
LMGM91
Explosive WeaponRPG-7
At the start of the mission Nikolai will open a brief case containing three different pistols, pick one of them up. You will then head on downstairs and encounter a bunch of enemies. Kill one of them with the pistol you picked up, run over to their body to see if they’ve got a different pistol to the one you are carrying. If not, then don’t worry too much as there’s plenty of weapons to use.
Then head on inside and you will find the
MP7 on the table and one of the enemies will be carrying a
1911 and another enemy will sometimes carry the
M19. Continue into the next room and the
Model 680 will be on the snooker table. Also, in this room an enemy will usually be carrying the
FR 5.6. Go though the kitchen and the guy on the stairs will have the
P90.
Head on back outside and you’ll be brought to a road that you have to fight your way down. In this area you can find the
FAL and the
AK-47 on some of the enemies. After you’ve jumped out of the window you will encounter some more enemies, check their bodies if you are missing a weapon. You will then head down an alley way with a guy on a high ledge and someone in front of you. Here they can potentially drop the
725.
Next area is the final room where there are a few enemies. The last two possible weapons you can use if the
M90 and
RPG-7 which is in the van you have to get into. Pick those weapons up and you should’ve used enough weapons to get the achievement. The achievements pops during the next cutscene.
This achievement isn’t really that difficult on Recruit, only hard part is remembering what you have used as you can’t stay in an area for too long.
If you are still struggling, then feel free to use this as a reference.
Call of Duty: Modern Warfare Hot Swap Achievement Guide