On my only "homebuilt" AR, the fit between the upper and lower is not so perfect and snug like my Bushmasters. I didn't build it myself, I bought it used.
There is just barely enough space to see light across the lower mating surfaces, looking from the side. But in the rear of the upper, where it meets the part of the lower that curls up toward the charging handle, there is about 1/16 inch space. It's enough that I can get a glimpse of the action spring when the receiver is pinned shut.
Is this a problem?
I've heard that GI M16s have a lot more play between the upper and lower than modern commercial ARs. I'm not bothered by the look- I just want to know if it's a sign of trouble or just a cosmetic matter.
I don't think it matters too much. My Colt is that way and it shoots fine.