In the Flight magazine of 1 July 1955 (https://www.flightglobal.com/pdfarchive/view/1955/1955%20-%200912.html) you may find an article which includes info on a twin 34mm AA gun. This is what the article says on that:

'By the entrance to the exhibition hangar are sited two pairs of the latest 34 mm fully automatic flak guns, radar laid at jet speed. Each twin-gun has a combined rate of fire of 900 rounds a minute, a muzzle velocity of 1,000 m/sec and a practical range of 4,000 m. Such ordnance further imperils the future of the low-level air-support business.'

So a single gun has RoF of 450 rpm.

Here is the photo of the gun from the article (its caption indicates that the gun was by Hispano-Suiza):

And at https://projectswisstank.wordpress.com/2016/03/28/tier-5-panzer-51-mit-34-mm-hswf-doppellauf/ there is info on what apparently is a self-propelled version of the gun depicted in the Flight. It is mounted (in my opinion with no major changes) on the chassis of AMX-13 (Panzer 51 in Swiss nomenclature). The post in the blog contains also some data, incl. that the gun was being clip-fed (I would think rather it was belt-fed) and says it was developed by Hispano-Suiza and W+F Bern. The caption of the photo below confirms that -  it comes from an article on the Swiss armament industry published in 2012 in the Revue Militaire Suisse (rms-001_2012_0__555_d.pdf ).

It seems to be quite an interesting gun, doesn't it?

So I wonder what ammunition it used. Was it 34x239 used in Flab Kan 38 water-cooled single 34mm gun of pre-WW2 vintage? Do you have any idea on its performance and - finally - on why the gun has never been manufactured and adopted in service?

Any info would be highly appreciated.

Piotr

Thanks for posting, not seen those pics before. A paragraph from my next book (with thanks to Bob Gerber):

34 mm HSS 834 and 40 mm HSS 840: 34 x 239 and 40 x 239B ammunition.

In the early 1950s the Kriegstechnische Abteilung or KTA (Weapons Technology Section of the Swiss Defence Ministry) requested Hispano-Suiza to develop a new heavy automatic AA gun intended to replace the 34 mm Flab K 38. The resulting gun was was designed in two calibres: the HSS 834 which used an updated loading of the existing 34 mm ammunition; and the HSS 840 which used a 40 mm version of the case, with a belt added to ensure accurate headspacing. Ammunition feeding was via a linked belt or five-round clips and the cyclic rate was an impressive 450-500 rpm.

The project was shelved in the mid-1950s. Oerlikon eventually replaced the Flab K 38 with their KD series guns (see below).

At https://naboje.org/en/node/13703# there is some info on a 40x270 cartridge by Hispano-Suiza. Original Czech description may be translated as follows: