2B1 Oka
Soviet 420 mm self-propelled tactical nuclear mortar
2B1 Oka (Russian: 2Б1 Ока) is a Soviet 420 mm self-propelled mortar. 2B1 is its GRAU designation. The mammoth 420mm SP Oka mortar system, which was originally intended to fire tactical nuclear rounds.
An experimental model was ready in 1957. Its chassis (Object 273), was designed and built by the Kirov Plant. Its 20 meter barrel allowed it to fire 750 kg rounds up to 45 km. Due to its complexity of loading it had a relatively low rate of fire - 1 round every 5 minutes. Field tests showed various drawbacks of the entire design (the recoil was too strong for many components - it damaged drive sprockets, tore away the gear-box from its mountings, etc).
Its development continued until 1960, when the idea of such overpowered guns (along with the 2A3), was abandoned in favor of tactical ballistic missiles, such as the 2K6 Luna.
2Б1 «Ока́» — советская самоходная 420-мм миномётная установка.

420-mm self-propelled mortar 2B1 «Oka» and its shell in Saint-Petersburg Artillery museum
An experimental model was ready in 1957. Its chassis (Object 273), was designed and built by the Kirov Plant. Its 20 meter barrel allowed it to fire 750 kg rounds up to 45 km. Due to its complexity of loading it had a relatively low rate of fire - 1 round every 5 minutes. Field tests showed various drawbacks of the entire design (the recoil was too strong for many components - it damaged drive sprockets, tore away the gear-box from its mountings, etc).
Its development continued until 1960, when the idea of such overpowered guns (along with the 2A3), was abandoned in favor of tactical ballistic missiles, such as the 2K6 Luna.
2Б1 «Ока́» — советская самоходная 420-мм миномётная установка.

420-mm self-propelled mortar 2B1 «Oka» and its shell in Saint-Petersburg Artillery museum