importing game object problems

i made a gun in blender and it looks pretty good, but when i imported it into unity some of the faces were invisible and the back side of them were visible when i looked at them from the other side. why did this happen and how do i fix it?

i think this is happening because i have faces brought back into the model, like a hole for a screw. is there any way to fix this without remodeling everything?

I don’t know much about Blender, but check your normals into that software, then check the importer into Unity (check that out). Finally, maybe the material is wrong ?

in the prefab settings click calculate in the tab for the importer coulm.