These weapons can be found ONLY in the campaign. Not multiplayer or zombie mode.
The KS-23 is a pump-action shotgun found only in the campaign. You can find it in various levels. For some reason Treyarch decided not to put it in the multiplayer.
The Model 1887 is a lever action shotgun, featured in Modern Warfare 2. It makes a cameo in Black Ops on the campaign level Vorkuta. You use it with one hand when riding a vehicle.
The SPAS-12 can be found in multiplayer and zombies. However, in the mission “The Defector,” you are given special incendiary rounds called Dragonbreath rounds.
In zombie mode and multiplayer mode you are given a crossbow with explosive bolts. In the campaign you are given a crossbow with scopes, regular bolts, and explosive bolts. You can find them on the missions “Executive Order” and “WMD.”
A bolt action rifle used by Russia during WWII. Found on “Project Nova” in the campaign. Holds 5 rounds per clip. There is also a scoped version on the same mission.
A full auto SMG used by Russia during WWII. Found on “Project Nova” in the campaign. Holds 35 rounds per mag.
A semi auto pistol used by Russia during WWII. Found on “Project Nova” in the campaign. Holds 8 rounds per mag.
A full auto SMG used by Great Britain during WWII. Found on “Project Nova” in the campaign. Holds 32 rounds per mag.
You can find the special Thunder Gun in zombie mode, but I recently discovered you can obtain it in the campaign mode.