I seem to be having a hard time putting the upper and lower together.  When I go to put the two together, the back of the upper will not completely close into the lower.  

The hammer is down and the safety is on.  The buffer pin does not seem to be in the way.  

There is a half centimer gap left to go for the upper and lower to fully mate so I can't push in the rear takedown pin.  Any suggestions?  

If i force the upper and lower together, there is still a little bit of gap left.  

Are they supposed to be this tight?

FYI:  It is a complete RRA upper with an olympic lower.

RRA has something of reputation for being a bit tight, you'll find numerous threads on ARFcom about it... Try this link for some help and info...

rbprecision.com/rb_precision_ar15_performance_tips_.htm