Headspacing, barrel tension/indexing, gas tube alignment, a strange spring or two, roll pins, sights, etc. are the issues with assembling an upper receiver from scratch. Also, it's really not that hard, but to do it all RIGHT, you really should have some special tools and gauges, blocks for vice, etc. Can you do it? Yes. Is it worth the time and effort? Not really, IMHO. I've done it, and I wouldn't do it again for my own tomorrow. I'd buy an upper or maybe re-barrel an upper at most. Just my .02....but the further down the road I go I like to wrok on things I like to work on, and assembling an AR upper is not something I see as worthwhile, in a practical sense. I'd much rather use that time and effort adding a rail system, grips, new sight, free-floating, better gas block, etc.....too far down the road, and you WILL have practically built an upper, but doing it a bit at a time for what's seen as an improvement seems much more enjoyable to me now. :mrgreen:
Here's a thought: Let the ducks work and decoy.