The 2S35 was a prototype twin-barreled 152mm self-propelled gun platform once under development in Russia. The system utilized the existing chassis and automotive components of the 2S19 "Msta-S" self-propelled gun (introduced in 1989 and born of the T-80 MBT) with an all-new turret structure. The key differentiating feature of the 2S35 when compared to other SPGs was to become its "over-under" gun arrangement featuring 2x 152mm gun systems. The guns were to be managed by a individual auto-loaders and could reportedly reach a rate-of-fire of 16 rounds-per-minute. A technology demonstrator was unveiled in 2006 but it appears that development of this vehicle was abandoned in 2010.
The selection of a two-gun approach was wholly unique to this vehicle and an interesting design choice when all comparable contemporaries have relied on single main guns for decades since World War 2 (1939-1945). Not only offering a substantially higher rate-of-fire, the dual-gun approach could yield improved first-hit capability (particularly with precision-guided ordnance), longer barrel service life and an inherent backup should one of the guns fail in action. Each gun is given a dedicated autoloader and reduces the operating crew. Conversely, the system was of a more complicated engineering design and thusly more expensive to procure in number. Use of the existing Msta-S chassis alleviated this to some extent logistically. All other self-propelled gun functions remained the same including tracked mobility and a fully-traversing enclosed turret assembly.
As in the Msta-S, the 2S35 would rely on a driver seated in the hull with a track-and-wheel arrangement in play. Power was to have been supplied by a diesel-fueled engine at the rear of the hull. Based on available imagery of what is believed to have been the prototype vehicle, the main guns were capped by perforated muzzle brakes and up to six grenade dischargers were positioned at the front-side turret walls (two banks of three dischargers each). A coaxial machine gun was assumed and support for an anti-aircraft machine gun on the turret roof was likely. The turret was to feature an operating crew of one (or perhaps two) and it is believed that the hull and turret would have houses up to 70 x 152mm ready-to-fire projectiles with on-call ammunition selection through an integrated Fire Control System (FCS). The vehicle was listed with an estimated road speed of 60kmh with a range out to 500km and a weight in the 50 ton range.
There were reports of the program's cancellation some years ago though this 2S35 project was once confirmed as active in February of 2014 - though this now appears very unlikely as the more conventional (single-barreled) 2S35 Koalitsiya SPG is set to be introduced into the Russian Army between before the end of the 2010s, succeeding the line of 2S19 SPG systems currently (2017) in service.
Some sources showcase the presented 152mm guns as 155mm units which is entirely possible for the finished product. The finalized design could have also fielded an entirely new chassis and hull apart from that as used in the Msta-S series. An 8x8 wheeled form was, at one point, another possibility for the twin-gunned vehicle.