Don't know why the problem occurs.
after smooth shading, this appears
I need an edge loop there for the sharp edge of the gun cut-out.
Don't know why the problem occurs.
after smooth shading, this appears
I need an edge loop there for the sharp edge of the gun cut-out.
The problem is that your normal directions have become mixed up. To solve, (in edit mode) select all, then pick Mesh > Normals > Recalculate outside.
mesh > normals > recalculate outside. – Christopher Bennett Jun 20 '20 at 19:55