There is PBR low-poly gameready model of heavy tank with railgun. Machinery is moved by a gravity motor.
Unity and UE4 engines allow to use packed textures to optimize memory usage (for example, in UE4 textures, metallic map is packed in B channel of OcclusionRoughnessMetallic texture).
For any questions related to the model or problems using the UE4 and Unity textures, you can ask me.