Newbie Modeling AR Lower in BobCad CAD

I've been through a few tutorials and made some changes to them and have a decent grasp of the CAM side and can see effects of changes and mistakes due to the simulator. All is good there. I like the tool a lot and want to switch to the tool as a CAD solution since I mainly have single components not assemblies. So far I am really digging V26.

But.. the only way to really learn a new CAD package is to use it to design something meaningful. Tutorials just get you acquainted. An AR lower is something dang near everyone knows and I don't have to describe what I am trying to do, it levels sets understanding. My current CAD software is 2D and quite different than BobCad.

I downloaded a 3D model to look at before trying it for reference. I assume doing it in 3D is more optimal.

1) Conceptually one can draw a profile of a lower. (example below)
2) add another layer and extrude it up the Z to the maximum thickness.
3) All of the holes and pockets are pretty straight forward.
4) Here is the challenge I see with new tools, how do I profile the section in red. (example below)
4a) maybe do a front view and subtract with a special shape?

As much detail as you can provide is helpful, I will do it and post it as I go with the general shape of the Lower but not to scale or anything. The drawing tools and techniques are what I am after.




Thanks for looking.