How to Shell between two circles

So i regularly use blender models, as references for learning the AutoCAD side of modeling. Today I am using the M4A1-S from the game counterstrike. I am running into a shelling issue, that i cant think of a simple solution for. I need to make a tapered shell for the Cone cut out. As I’m typing this i realize i could have drawn the geometry of the cone then revolve it around the axis and subtracted. Is there an easier way? if not i think that is my solution.

2 ways come to mind.

One, extrude the circle inward cutting inward. Select the bottom and use scale tool to scale it down

Two, make a plane to make a revolve profile for the cone to subtract with. Xray mode helps. More accurate than method one