I just bought another AR15 which makes it AR15 #3.  It's a preban M4A2 Oly Arms lower with FN 16" Hbar upper.  I got it for a very good price that I could not pass up.  Anyways, I noticed a small gap or space between the upper receiver and lower receiver where they meet near the back.  Anyone know what would cause that?  I'm thinking about selling the FN upper anyways and buying a Bushy or RRA upper.  Any advice would be appreciated.

How much space are you talking about?  I can see a thin line of light through my bushy. Is it loose?

You're describing a normal variation.

There's not too much space but it is noticeable.  It is not loose.  Probably nothing to worry about.

Like [b]para-bush15[/b], I’m also wondering how much space you’re talking about.

And, no offense, but the mention of a FN upper on the open market is always good for an argument!!

Apparently FN is legally prohibited from selling such parts.

Supposedly, vendors sometimes represent uppers as FN when in reality they are made by third parties that supply uppers to FN.  However, these might be uppers that don’t meet FN’s specs.

Also, supposedly vendors sometimes simply pass off uppers as being FN when there is no connection at all.  Such uppers might not be of especially great quality.

Thus your upper might be a little out of spec.

However, Oly also had a reputation in the past of not always being completely in spec.

Since you’ve got a couple of other AR’s, you might want to mix and match uppers and lowers just to try to find out if any specific part is out of spec.  You certainly don’t want to buy a new upper and then find out that the Oly lower is out of spec.

Just my opinion, of course.  Hope it helps!

Edited to add that I started this reply before you posted the above (and I'm amazed that it apparently took me a full half hour to type a dozen sentences!!!).