I found 2 guns which seem to be about as rare as it gets.. 

First were the original prototypes for the P90.  They had no stock,  The idea was to point and shoot.  They still kept the sideways magazine though. 

https://medium.com/war-is-boring/some-pretty-weird-prototypes-preceded-the-p90-submachine-gun-e57181055cea

Incidentally if you Google MP7 prototypes,  It shows the original was a much smaller,  holster-intended gun. 

Lastly was the Soviet AO27,  basically an AK shooting darts.  Apparently it didn't fire the famous 10x54mm flechette,  instead firing a new 3mm dart. 

It was cancelled due to poor dispersion

https://guns.fandom.com/wiki/AO-27

anyone else got a gun that nobody has heard of? 

Going slightly off subject,  I have found the most ridiculous gold-mine of information on obscure 60s weapons,  all by the same guy. 

Detailed analysis of dart-firing AK

https://en.topwar.ru/563-yeksperimentalnyj-avtomat-shiryaeva-ao-27.html

SMG which shoots bullets shaped like an 'H'

https://en.topwar.ru/158664-mnogokalibernaja-pulja-i-pistolet-pulemet-specialnogo-naznachenija.html

I googled 'Topwar arrow bullets' and it lists many articles with almost insider-like knowledge of U.S. projects.  You will need Google translate turned on

he's also the only info source on the rare steyr IWS sniper

https://en.topwar.ru/17451-snayperskie-ruzhya-steyr-amr-i-iws-2000.html

Just google 'Topwar' followed by whatever subject your after

Topwar is a junkyard of military related info, and anything posted there must be taken with a grain of salt at the very least

But if you want guns that are interesting and unique...

British parker hale PDW.  Invented by a non brit working for Bush man,  then purchased by parker hale,  who had previously only worked on bolt actions. 

Had a rate reducer in the grip. Could take screw- on barrels

I'm glad you said! With english as my first language I can sometimes find Russian to be a bit crptic but this guy rambles for paragraph after paragraph of garbage.

Many Topwar texts are crap even when posted in their native Russian language, and further use of Google translate does not help them any.

So, topwar is the Russian equivalent of Business Insider?;)

Also the G11 had originally been designed by mauser, with a 10 rpund rotary magazine.

Another protoype by IWK had 3 barrels, and could fire a volley of all 3 shots