I was also kind of unhappy with overall progression and especially the limited (or rather exclusive) manufacturing vs. buying options for tritanium ammo - why can x-com manufacture their own ammuniton for some weapons, but not others? Why can't we outsource things to BlackOps as well?
So I just added options for that to my XCF submod. This also ties in with my upgraded/alloy missiles, which can be produced as well as outsourced later on.
I can partially understand Solarius in doing things this way, apart from the fact that having manufacturing+buying options for the same items is kind of redundant. I mostly did this for fluff/immersion reason and because buying items is simply more convenient than having to manufacture+stockpile them.
If something like this bothers you too much - mod it. That's the great thing about OXC.