Hi,
I just just installed a new upper on my cast  lower and the fit was TIGHT.  Both the pivot and take down pins could move freely easily without the upper. The front pivot pin pushed through the upper and lower nicely. But, the take down pin was a PITA. The the holes would not line up. It seemed the upper and lower would not come together close enough near the rear of the receiver.

So eventually I had to soak the holes and pins with CLP to loosen them up. Well, the good news is I got the pin through finally. But, I had to SQUEEZE the upper and lower together with my GI Joe Kung Fu grip to get the holes to align.

Am I going to have to use a rubber mallet every time I need to take down my rifle or is there a way I can loosen the tension on the take down pin? Is it safe to relieve a portion of the holes to allow easier movement of the pin?

Thanks Brad

Try a 1/4" rotary rasp.

If you know how to remove the pin you can adjust the shape of the pin to fit the hole. I always alter the cheapest part which is the pin. Doing the pin keeps the upper and lower holes the same size in case you want to move the upper or lower between guns.  
Sometimes the pin will loosen after a while from shooting and cleaning the gun a few times and nothing has to be done.

While altering the cheapest part is good advice I try to fix whichever part is wrong. Given that your lower is cast I'm assuming it is wrong.

Is it an OAI lower?

It seems that the responses may have gotten a little off track a little based on what you posted above (edited to show only relevant symptoms/solutions). Before you alter the holes or the pin...a little clarification please. When you squeeze the back of the upper, do the holes line up?

I sounds like the problem usually associated with the tight fit experienced with RRA lowers. The rear pin lug on the upper may be binding against the inside rear of the lower. Look at the lug and the area inside the lower near the takedown holes. Are there fresh scratches or scoring marks? If yes, then it should loosen up after repeated assembly/disassembly and a few hundred rounds without any modification.

neilfj is right about the RRA lowers.  I recently build a RRA lower and was convinced the lower was out of speck because neither of my uppers (both new Bushys) would fit the lower.  After some research, I learned I might have to apply a good amount of pressure to get the upper and lower together.  It was suggested I might even need to use a rubber mallet.  I ended up turning the rifle upside down and had a friend hold the barrel steady while I pressed down VERY hard on the stock until the pin would fit go into the hole.

After shooting a few hundred rounds the upper and lower are still tight, but now fit together without applying so much force.  I didn't need to modify anything.

I just built a new AR on a RRA stripped lower, and used a DPMS upper.  It exhibited the same "tightness" when fitting to the lower.  I put a RRA upper on it and it fit fine.  After examining the RRA upper and DPMS upper, it appears that the rear pin hole on the RRA upper has the two rear sides tapered in a bit (my DPMS uppers are all perfectly squared).  I decided to try a little bit of sandpaper on the DPMS part and just filed it down a very small amount to taper in the sides a bit (rounded them off).  It took literally 30 seconds of sanding (manually - no power tools here) using a very light grain.  I then tried the DPMS upper in the RRA lower and it fit perfectly - no longer tough to get in!

If you need pictures of what I meant, I can take some tonight - just PM me or email me.

I figured I'd just make this available to everyone...  I snapped some pics of my RRA upper, my untouched DPMS upper and my "fixed" DPMS upper, and how I sanded the DPMS upper to fit into my RRA lowers.

I hope it helps!

[url]http://www.airsoftarizona.com/fix/[/url]

I've also seen this condition caused by a buffer tube which extended into the lower just a little bit too much, causing the upper to bind as it was almost closed. This is more common with collapsable stock buffer tubes, but can occur with fixed stock tubes too.