There is PBR low-poly gameready model of soviet anti-tank gun. The model was made according to my own design.
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.