Lock the front take down pin in place, drop the upper down as far as you can get it, and give me a photo looking through the rear take down channel to the back take down lug channel of the upper receiver (as close as you can get them to align).


I want to see if the problem is the channels are off front to back, or the problem is the upper receiver can not be lowered enough on the lower receiver isntead.


As for the rear take down pin being tight to start with, it kind of what I want in a upper to lower fit to begin with, since a little lube and a few shootings and cleaning will allow the rear take down pin to mate with the channel, and still have a tight fit down the road.  Hence easy slip of the take down pin line to begin with/the rear take down pin sliding way to easy through the rear lug channel, means by the time you get a few  shooting and cleanings, the upper to lower receiver fit is sloppy, and then you trying to figure out way to tighten them up again.  

Simply, use to be a time when the RRA uppers to lowers where a very tight fit (ended up just like you have now) and because of such ideal tight lock ups of the two, was the upper and lowers that I used to build NM rifles.  I bring this up, since you may be looking a gifted horse right in the mouth, and don't realize it isntead.

P.S, a chomp stick makes for a great punch to drive the rear take down pin over to the unlocked postioin, since it will blow up/shatter before you can do any harm the receiver.

Need a see a photo like this of your upper and lower mated together.  Hence far enough away that I can see the fit of the two at the back, plus how the upper lower lug channel is matching up the receiver channel.

Looking at the above pic, it is hard to tell if it's the upper receiver or lower receiver that is slightly out off in its machining, or a combination of both which is creating a stacking issue.  You state that this project was a second rifle, have you tried fitting the upper on your other lower, and tried this lower with your other upper?  

There could be a couple of different areas that might be the cause, or help remedy the situation.  When you go to close the upper down on the lower, is it that the upper is stopped from going down at about the back of the magazine well area, meaning that the top of the lower receiver and bottom of the upper receiver are not mating correctly on the same plane?  Is it that the upper receiver seems too long in length or lower receiver is machined too short, where the back of the upper receiver won't come down enough to allow the hole in the rear takedown pin lug of the upper receiver to properly line up with the lower receiver?




Looking at these above pics, there are signs of where the rear takedown lug corners on the upper receiver are rubbing inside of the lower receiver, which is possibly binding and not allowing the upper to come down all the way.  If you think this is the cause, you might be able to use a safe edge file at the 90-degree back corners of the lug, just barely removing a very little of the corner (kind of making it into 45-degree) so the lug goes down into the lower receiver easier.  


CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

DpARmiami,  DON'T TOUCH OR FILE ANYTHING ON THE RIFLE OR UPPER!!!!!

As Sully pointed out, when you look into the lower receiver back shelf, you can see that the back edges of the upper receiver take down lugs are slightly binding on the side voids of the back shelf walls and causing you know to have to put a extra pressure on the upper to get it push down so you can get the two locked together.  In a  couple of hundred live fires and a few cleanings, the edges of the upper receiver rear take down lug that are binding on the lower receiver shelf walls, will peen inward on there own, and the this will allow the upper receiver to mate all the way down so you can slip the rear take down pin over much easier, while still have the lock up between the upper and lower TIGHT!!!!!


So for right now, you will have to use a little extra pressure pushing the upper down on the lower so you can drive the rear take down pin over and lock into the place, but truth me, in a few hundred rounds and a few cleanings alone the way, the two edges of the lugs back edges that are binding now, will peen inwards, and this problem of having to use extra force to get the upper locked down on the lower will not longer be a problem (but still have a tight lock up between the two).

So in the mean time, a little grease on the back lug edges where they are binding now, some on the rear take down pin and the lug channel, and you are good to go to break the rifle in.

Hence the only time you really want to break out a file and remove metal,  is if the bottom of the upper receiver take down lug is too long for say a high shelf receiver, and it binding on the bottom of the back shelf isntead.  This is not the case here, and isntead you have what RRA does normally, and that is to make the rear lug wider than normal, so when the upper is locking down on the lower via the rear take down pin, there will be no side to side movement of the upper to lower isntead.

So again, your looking a gifted horse in the mouth, and not realizing it instead.  Hence you break out the file now, and in a 1K of rounds and cleanings, will try to be coming up with a way to tighten the fit back up, to remove the side to side movement of the upper to the lower isntead (hence peen'g the back of the lug  to get the back side edges tight to the side walls again)..

http://www.ar15.com/forums/t_3_66/282974_Why_won_t_my_new_upper_receiver_lock_close_on_my_new_RRA_lower_receiver_________.html