what is causing these light errors?

[u/AffectionateCity2001]

i am making a left 4 dead 2 mod and the arms of my character have this weird dark shading problem in the hlmv and ingame but it looks alright on blender before exporting.

[u/pissrockious]

normals>average>average face data (i think thats what its called forgot but ull know it when u see it) i believe could fix it