 Armour protection of the turret was significantly improved and it was increased in size again, reaching a width of 3.12 meters (without explosive reactive armour); the widest rear of all welded turret Soviet tanks.These modifications had the same Malakhit explosive reactive armour as the fourth modification, except that the reaction plates were made of steel instead of titanium.Uralvagonzavod undertook enormous efforts for this tank to be adopted, but in the end after successful trials proving high combat and technical potential, but the tank was not accepted in part due to old ammunition of 2A46 not being reusable by 2A66, making costs prohibitively high.