Helping BIL put together his first AR.  Finally got everything together yesterday, go to put the upper on the lower.........and........ruh roh!  Pins wont go through the holes.  I can rotate the upper forward, and get the pivot pin through, but then when i rotate back down to put take down pin in.....the upper won't seat flush to the lower???  about an 1/8" gap when it gives me resistance.  I can apply a LOT of pressure and it closes to a 1/16 or so, and i can get the takedown pin in one side. But that is it.  Even some healthy love taps with the rubber mallet don't get it through.

It is a stag lower, and it is the problem.  Upper fits on two other lowers, and those uppers give same result on this lower.  Can i take my 80% jig and just re drill the holes, assuming this is the problem?  I really can't see what is making contact to prevent the upper from sitting flush in the lower???  What am i missing??

Attached File

your choices are to re-drill holes, sand/undersize your pins, buy a set of undersized pins and pray, or send the lower back ($$$), or keep swapping uppers until you find a really loose one.

Wasn't even thinking correctly....

How do i redrill the holes if they are the problem?  The jig indexes off of them, correct? (5d tactical pro)

I was thinking about it, and would it make sense that getting the pivot pin in....it isn't seating correctly b/c its binding on the pin b/c its out of line, when i rotate down? This is all i can come up with?

The rear takedown pin goes through when the upper is off the lower, correct?

That being the case, I'd just dremel out the hole on the upper until the pin will go through to the other side.  One of those small round sanding drums or a rat-tail file will do the trick.  
Place the upper on the lower with the front pin out and you should be able to eyeball where material needs removed.  Failing that, just look inside the hole on the tab of the upper, you should be able to see where it's rubbing/scraping.  
You may have to sand/file a bit off the sides of the tab on the upper it it's too tight to close on the lower, too.  You should be able to see where it's rubbing and just take it down a little bit at a time until it fits snugly.  
That way all of the removal is on the inside and out of sight.  Touch it up with a black sharpie if it bothers you and you're good to go.  
The oil from the lube on your BCG will keep anything from corroding.

**I've had to do this on a couple of builds.  Usually they're juuuuust off enough to put the pin in a bind.  I wouldn't widen out the holes on the lower at all, ever.

Look at the upper lug at the rear two corners. Sometimes they hit the curves in the back pocket of a Stag/CMT made lower. CMT did not used to make the side in cuts at the rear of the pocket and some upper lugs hit at the back corners. when closing. If it is you can see two lines in the back curves of the lower where the lug is rubbing. There a tacked thread about it from long ago if it turns out to be the problem.https://www.ar15.com/forums/AR-15/Why-won-t-my-new-upper-receiver-lock-close-on-my-new-RRA-lower-receiver---/66-282974/

If you screw a stock tube in past flush with the inside top edge of a lower it keeps the upper from closing also.

I would say your lug on the upper is wider than the cut out on the lower, which is not all the uncommon, I have had to file upper lugs to fit certain lowers before.  In this situation, a sanding drum and a dremel are your friend that way to can get it to fit that lower, whether you file the lug of increase the width on the lower lug area, often times it is just the back two corners on the upper.

I'd bet these guys are right.  But what the hell is the "Stag lower" and "5D tactical" doing in the same thread.  Stag making 80%'er now or are you thinking of taking a router jig to a factory lower?  Don't re'drill or ream the holes.  If anything takes more than a minute or 2 with a very light handed file, I'd be contacting the manufacturer before any real damage is done.  You say the upper works on other lowes and other uppers do not work with this lower,  remove as many parts as possible to check this upper and lower without any other obstructions. If anything does require filing/sanding go very little at a time and work on the cheapest part first. Don't file or sand any part where tollerances are critical like take down pin holes or hammer trigger pin homes  Also, don't drill/ream holes until the upper and lower mate properly.  
Before you do anything to ruin any part worth more than you're willing to replace, think twice and or find somebody who's done it before.  Preferably more than once.  I really like working on guns.  I really dislike fixing somebody else's bubba'ed up mess.

Tig,

I have a couple of Anderson's that I bought two weeks ago,that I had to work on to get any of my uppers to fit correctly on them, I am not surprised at anything I see these days with so many being sold.

Is your buffer tube screwed in to far?

Ok...First, thanks to all for the replies.  See if i can catch this all in one post...

The pins do go through lower just fine without the upper on.  Simply setting the upper on top of the lower and looking through the holes, it appears that the upper sits too far forward by just a few mm.  Looking at it, the buffer tube might protrude a HAIR past the upper, but it depends on the angle i look from.  I don't think it is the problem.  I did not notice any marks from the rear lug rubbing but i will check that more closely as that would seem to be a likely culprit.

Assuming that is the case just knocking down the lug on the upper a bit is the easiest/best course of action?

And yes, this is a factory stag lower, and i was thinking i could touch up the holes with the 80% jig....but that was when i was not thinking very straight....

This is my first guess too.

Funny this popped up!  Just got a chance to get back in to this this morning. After some looking and fiddling... I decided to back the buffer tube one turn just in case. And what do you know!  Learn something new every day. It was still VERY tight and the pivot pin still require a tap with the mallet.... But we're running fine now!

Thanks for all the help!