This is my take on the L-34 gun, from Finnish books and a Tampella brochure, as included in my forthcoming cannon book:
20 mm Lahti and Tampella L-34: 20 x 113 / 20 x 113B ammunition.
The first prototype of this cannon emerged in 1934 and after some modifications the Finnish Coast Guard ordered ten L34/II in 1935 to arm new patrol boats. These recoil-operated guns were made by the state-owned VKT and delivered with L/60 barrels and 15-round box magazines, although 45-round flat pan magazine and belt feeds were experimented with. With a weight of 48 kg and a rate of fire of 300-350 rpm the gun was an unremarkable performer. The 20 x 113 ammunition was unique to this gun.
No further sales were achieved by VKT but another Finnish engineering company, Tampella, negotiated manufacturing and export rights. They slightly modified the gun and its ammunition, designing a belted version of the rimless case, and promoted it abroad. Two examples of the Tampella L-34 were made and saw wartime service with an AA unit of the Civil Guard at Tampere, but no sales resulted. This weighed 40-60 kg, had a 15-round box or 32-round pan magazine and fired at 325-360 rpm.
My Lahti ammo entry reads as follows (as yet, some unanswered questions - any answers welcomed):
20 x 113, 20 x 113B Lahti
These rival the 20 x 105 as being among the least-used service cannon rounds. The L-34 “boat gun” was developed in Finland in the mid-1930s by Aimo Lahti. It was a light automatic cannon made by VKT and fitted to ten patrol boats of the Finnish Coastguard. The cartridge case for this gun is brass, rimless and tapered as well as slightly bottlenecked. Projectiles, which have brass driving bands, are secured in the case by a segmented crimping groove in the case neck. Overall length of the complete round is 177 mm and the weight is around 300 g with a 148 g AP projectile, fired at 700 m/s; a lighter HE loading was also made. When firing AP shot penetration was 30mm/300m/90º or around 20mm/300m/60º.
Subsequently Tampella took over production and developed a belted version of the cartridge. A brochure for this gun gives an overall round weight of 275 g, with both HE and AP-T projectiles weighing 136 g and fired at 800 m/s; a significantly more powerful loading than the original.
Lahti also developed a number of other 20 mm rounds. The complete list of all known Finnish 20 mm cases is below (info from Bill Woodin). The purpose of the experimentals (in italics) is unclear.
20 x 75B (rim 26.0 mm, body 25.1 mm)
20 x 81 (rim 23 mm, body 23.5 mm)
20 x 97B (rim 25.4 mm, body 26.2 mm)
20 x 98B (rim 27.5, body 26.0 mm)
20 x 99B (rim 25.0 mm, body 25.6 mm) very close to 20 x 97B – may be the same
20 x 113 (rim 26.1 mm, body 26.1 mm) early version of beltless ammo for L-34 gun?
20 x 113 (rim 27.9 mm, body 28.0 mm) later (more common) version of L-34 beltless?
20 x 113B (rim 27.5 mm, body 26.0 mm) Tampella version of L-34, also aircraft cannon prototypes L-35 and L-36?