Wow, you really have got an amazing attention for detail. I dont see any obvious flaws with it, but you are missing the pins for the trigger gaurd and I think you need to make the pins holding the lower and the upper together protrude out a bit more. Also, I cant tell if you have changed the area around the forward pin (holding the upper and the lower together) or not, but from your earlier pictures it looks a bit wrong. If you would like detailed pics, I reckon I could PM you some.
Also, could you provide renders from different angles of the gun? It makes it a bit easier to spot mistakes. And finally, word on the street is that the default background sucks so trade it out for white or black or something.