Hi Andrei, yeah it looks like there will be some pretty tricky transition areas in there.

For your red area, I'd think you would want to have it initially a big unbroken piece with a surface going across the whole area like this:



Then probably make a second surface that will cut the other one like so:



After those are trimmed with one another you'll then try to put a fillet where they intersected.

Looks like on this rifle you may want to have several layers of things that are similar to that, with chunks scalloped out from a bigger surface by having a kind of curved cutting surface that takes some of the larger block's material away, does that make sense?

- Michael