PKM you can find in the first and the last mission has a unique iron sight and a black grip.
M19 you start with in Piccadilly has a unique gray color and drops of water on it. It’s the only weapon in this mission that’s actually wet because of rain.
In the same mission you can find a full auto fal which is impossible to get in multiplayer.
In the mission Embedded you can put an oil filter suppressor on any gun that you can find. It’s important to note that this suppressor can be put only on pistols in multiplayer so this mission has many unique guns that you can make yourself. The coolest one is the revolver that you can find at the beginning of the mission on a box if you turn around and walk forward a bit. It’s cool because in multiplayer it can’t be suppressed at all and it has a unique shooting sound.
M13 you start with in Clean House has a unique barrel with no side rail and a rope wrapped around it. It also has tape wrapped around at the end of the mag.
In the mission Highway of Death Al Quatala fighters can drop unique AK’s with taped barrels, magazines and unique grips.
In the Church in the mission Going Dark you can find a unique Model 680 with a Colossus suppressor. This suppressor can’t be put on this gun in multiplayer.
And that’s all I know. Feel free to comment any guns that I forgot to mention in this list!