Based on the well-known GAZ-AAA 2-ton lorry, the BA-6 armored car was introduced at the Leningrad Izhorskiy plant in 1935 as the production replacement for the BA-3 after only 160 BA-3s had been manufactured.
Though a good overall design, the BA-3 was at 6.000kg considered too heavy and the BA-6 was essentially a minor production modernization of the earlier design, nearly 1.000kg lighter and with a strengthened rear suspension and modified transmission.
The BA-6 was aimed with the same basic armament as the BA-3 consisting of a 45mm M-1932 tank gun and co-axial 7.62mm DT machine gun in the T-26 turret and a secondary 7.62mm DT machine gun in the hull superstructure alongside the driver.
The BA-6 had better battle endurance than its predecessor, with sixty rounds of 45mm ammunition for the main armament stowed internally as against forty for the BA-3.

The BA-6 was built on a shortened GAZ-AAA chassis and was powered by the same GAZ-AA engine as the BA-3, the power output remaining 40hp.
The redesign of the BA-6 allowed for an improvement in armor to 9mm in vulnerable areas while achieving an overall reduction in weight to 5.120kg.

The vehicle's improved power/weight ratio resulted in better all-terrain performance, though the cross-country speed was reduced.
The use of bulletproof GK tires reduced road speed to 43 km/hour and range to 197km.
Several minor modifications were made to the BA-6 as production continued. The first few vehicles manufactured retained the small rear access door at the right of the fighting compartment rear which was a distinguishing feature of all BA-3s. Without reference to the lack of rear door, the BA-6 is almost indistinguishable externally from the earlier BA-3.

The BA-6 saw extensive combat service, with its combat debut being during the Spanish Civil War.
It served with Russian forces in the 1939 Khalkin Gol campaign against Japan, in the invasion of Poland, and during the 1939-40 "Winter War" with Finland before serving in the opening stages of the 1941-45 war with Germany.
Due to large numbers of BA-6s being stationed in the Far East Military District when Germanv invaded Russia in June 1941, these vehicles arrived late into the Western theater of operations, hence some BA-6s survived into 1943-44 and longer, while contemporary Russian tanks such as the T-26 light tank were mostly destroyed by early 1942.

However, many BA-6s were captured by the German Army and pressed into German service as the Panzerspahwagen BA-203 (r).

A total of 386 BA-6 armored cars were produced between 1935 and 1939. including a small number of mpdernized BA-6Ms.
The BA-6 series was replaced on the production lines at Izhorskiv bv the BA-10 from 1938.

Source : James Kinnear, "Russian Armored Cars 1930-2000", Darlington Prod.Inc., 2000 [ highly recommended ! ]

The only known variant of the BA-6 which served with the Red Army was the BA-6 ZhD rail scout conversion of the BA-6 armored car fitted with flanged ZhD road wheels and built in small numbers in 1935.
The BA-6 could attain a reasonable speed of 55km/hour on rails but had a limited rail range of 110-150km.
The BA-6ZhD had a combat weight of 5.900kg, due primarily to the steel wheels but also in part due to a larger amount of ammunition beina stowed.
To mount the BA-6ZhD system the front of the vehicle was raised using a tank hydraulic jack mounted under the vehicle hull.
The wheels were changed out and the process was then repeated for the rear wheels, with the rail wheel mounted to the inner rear wheel only.
The operation of changing the wheels took about thirty minutes to complete.