The Thundergun is one of the most useful weapons in Zombies Mode, especially when another player is downed. It can clear huge amounts of the undead with minimum effort, which can help the player reach the downed player extremely easily. It can kill multiple zombies without hassle and can save the player's life, but the downside is its short range. Despite being unusable, the Thundergun also makes an appearance in "Five" in the weapons-testing lab, suggesting that the Thundergun was found by the United States and was being reverse engineered by the Pentagon.

The Thundergun was originally thought by Richtofen to have been created by Dr. Maxis and that Maxis had been deliberately hiding it from him as he grew to not trust Richtofen, although this could mean that Richtofen did not trust Maxis and became paranoid. Richtofen even referred to it as the DG-3 (meaning he considered it to be the Wunderwaffe DG-2's successor). However, one of the radios in the map, Ascension reveals the true creation of the Thundergun. It was a Russian weapon codenamed "Project Thunder" and was being worked on by the Ascension Group under Gersch's direction. Gersch knew about its range problem and was trying to fix it. He also knew about its small ammo (power cell) count.

I say the gun is from Narnia, cos random shit = Treyarch

I'm thinking it has to do with BO campaign.

Such as Dragovich and Steiner were both on the opposite side of WWII until Project Nova when Dragovich attempted to kill off reznov to bring trust with the Steiner. Also In the Cold war, The russians actually used Nazi Scientists to help build Sputnik AND Rockets for the Space Race.
I.E. This could be for the arms race too.
Germany was both sided (Berlin wall) and the scientists wanted to give away their projects (America claimed the raygun, Monkeys and Teleporter, whilst the Russians claimed the DG2 and many unknown Weapons from 935)
But what about the thundergun/DG3?

Maybe because Maxis in the recordings requested fundings to make a leap for bigger projects but the Nazi's were loosing so they didnt have enough funds to Finish the DG3. And because it was Edward's Idea, Maxis didnt want to dissapoint edward saying that the DG3 couldnt be finished.

So when In Executive Order, Dragovich Had the Nazi Scientists relocated from (or to) The Ascension Program. Meaning the Nazi's had the advantage to work on the DG3 with the fundings from Russia which is why it appears in Kino and Ascension (In the future/Present day). Also The DG2 was originally going to appear in Kino due to unused quotes and the DG2 model in the files of BO1.

But In Five America has stolen nazi documents found in the pentagon so they were trying to make the thundergun DG3 themselves.

Thats just my theory. (my "game theory" )

The Ascension Program

Germany was both sided (Berlin wall) and the scientists wanted to give away their projects (America claimed the raygun, Monkeys and Teleporter, whilst the Russians claimed the DG2 and many unknown Weapons from 935)

But what about the thundergun/DG3?

Maybe because Maxis in the recordings requested fundings to make a leap for bigger projects but the Nazi's were loosing so they didnt have enough funds to Finish the DG3. And because it was Edward's Idea, Maxis didnt want to dissapoint edward saying that the DG3 couldnt be finished.

Thats just my theory. (my "game theory" )