Modeling Related Question How to model this piece?

I  am new to blender, I have tried many times but failed to get this shape of the piece especially the curve  inside the mesh.



Its a sniper model the name is Ai AS50  and here is what I have done so far as i wanted to block out this piece I have attached my work as well as the reference that I am using
I would use boolean operations to model the curved insets on the gun that you circled:

if you have a solid part of the gun:



Model a second object that is the shape you want your inset to be:



Position your second object to overlap your gun. Then add a boolean modifier set to difference to the gun shape, and select the second object. After hiding the second object, you will easily get a curved inset: