Let’s talk about the BMP, specifically the BMP-1 and BMP-2. The BMP-1 was pretty revolutionary when it first appeared, and was extremely influential. Interestingly, the design requirements go back to the early 1960s, when Soviet Doctrine came to embrace tactical nuclear weapons and chemical weapons. This led to a desire for a vehicle that would be highly mobile and allow infantry to fight from inside in order to minimize exposure to the expected hostile battlefield environment. By 1967, Soviet interest in heavy use of tactical nuclear and chemical weapons had waned, but the design requirements were set (go figure).
The BMP-1 used the powertrain from the PT-76, came with amphibious capability, and provided NBC protection for the crew of three plus the eight dismountable infantry inside. The infantry were provided with NBC-sealed firing ports for their Kalashnikov assault rifles and machine guns, and had roof hatches to operate weapons with a backblast, like RPGs. These ports were arranged four per side and two aft. The turret of the BMP-1 had a 73mm smoothbore gun that fired the same projectiles as the standard infantry recoilless rifle, a coaxial machine gun, and a rail for Malyutka ATGMs. The turret distributed some platoon/company level support weapons down to the carrier vehicle. This turret was a one-man affair, as Soviet doctrine of the time had the commander dismount with the rest of the infantry in the back to lead them.
The BMP-1 had a number of notable shortcomings. The lack of armor is one, and the small size meant that any hit would strike important, flammable things. The commander-outside-the-turret arrangement made fire control difficult, and forced the gunner to work as an ersatz vehicle commander when the squad dismounted. Wartime experience in the 1973 Yom Kippur war was decidedly mixed. The Egyptians were satisfied with their BMP-1s, which were used to support missile-equipped antitank infantry teams. The Syrians used their BMP-1s in head-on attacks that left them to be smashed by Israeli antitank guns.
The effectiveness and widespread use of antitank missiles in the Yom Kippur War was also a problem, since even the most rudimentary missiles outranged the 73mm gun on the BMP-1. A quick response to suppress an antitank missile team (and screw up their command-guidance concentration) would also be hindered by the separation of commander and gunner. Hence, the BMP-2 received a two-man turret with a 30mm gun that had a much more reasonable trajectory.