In Metal Gear Solid 3: Snake Eater, the weapon being developed at the Sokolov facility is the Shagohod.
The Shagohod is a Soviet nuclear-capable tank designed by Nikolai Stepanovich Sokolov. Its unusual design combines massive tracked propulsion with a rocket-assisted system intended to give it intercontinental range. The project is one of the central military objectives of Operation Snake Eater.
The Shagohod is often mistakenly called a Metal Gear, but the game distinguishes it from the bipedal nuclear platforms that define much of the series. It is a predecessor in the broader history of strategic walking or mobile nuclear weapons, while its own design uses large conventional tracks and propulsion technology. Its development also explains why Sokolov is considered so valuable by both Soviet and American interests.