I have a flattop upper receiver from CMMG.  I mated it with a Stag lower, and my only complaint with the completed carbine is that the upper-lower fit is extremely tight.  When putting them back together after a field strip, I have to press the two together with plenty of force before I can press the rear takedown pin through.  I haven't determined if it is the rear lug on the CMMG upper that is large, or the shelf on the Stag lower that is a little tall.  When I return home, I intend to test both the upper and lower assemblies on a couple of other ARs to determine which receiver is the culprit.  

I posted this information before, but I was looking for ways to alleviate the tight fit then.

With this in mind, had anybody had experiences with tight receiver fit of either a Stag lower or CMMG upper?

I have been told that it will loosen up after being fired plenty, but I am not satisfied with this as a solution.  As tight as the fit it, there is no way that it is going to loosen up to a degree where I can push out the rear takedown pin with my finger, which I have always been able to do with the rifles and carbines I have encountered in the military.

Usually one sees posts about the upper/lower fit being too loose, and in need of an Accuwedge

Some folks strive to have your situation.

You could always use this as an excuse to build another lower. See how these guns multiply.

I think Bushmaster makes a little delrin pin pusher that will fit on a key ring.

My Guess is it's the Stag lower. Stag is basically CMT. CMT makes lowers for a number of other companies, one of which is RRA. RRA lowers have a reputation of having a really tight fit with most uppers. Granted, you don't read about to many people having that problem with Stag lowers but then again, you don't really hear it about RRA lowers anymore either.

The good news is it's not a big deal. Open it and slam it closed 5-10 times then go shoot a few hundred rounds through it and I bet it fits perfectly.

My CMT upper and Stag lower are really tight. Tight is good. Its harder to take apart, and or put togehter, but tight is good. I have seen video of the Armalite guys using plastic and or rubber hammers to push their pins in. There are several products on the market now that make the upper to lower fit tighter. Many find this a desireable thing.

I wouldn't file anything off, or cut anything off, or anything... once you do you can't put it back on.

Its not a problem for me, and of course it does not adversly affect function. I just got a small plastic and rubber hammer from sears to push the pins in, and one of these:



for pushing them out.

www.bushmaster.com/shopping/gunsmith/imp-1.asp



It will probably loosen up with use. For now don't worry about it. Get the tools if you want an easier way to pull it apart.

The hammers are here....

www.sears.com/sr/javasr/product.do?BV_UseBVCookie=Yes&vertical=TOOL&pid=00938298000





Neither tool will mar your finish.

Quoted: My Guess is it's the Stag lower. Stag is basically CMT. CMT makes lowers for a number of other companies, one of which is RRA. RRA lowers have a reputation of having a really tight fit with most uppers. Granted, you don't read about to many people having that problem with Stag lowers but then again, you don't really hear it about RRA lowers anymore either. The good news is it's not a big deal. Open it and slam it closed 5-10 times then go shoot a few hundred rounds through it and I bet it fits perfectly.

I will give this a whirl when I get back.  The main reason that I don't like it being too tight is the fact that it prevents me from stripping it without tools of some sort.  It's kind of like needing a wrench for the full-length guide rod on your 1911  

Both of my Stag lowers one w/ M1S upper and one w/ Stag upper are/were both extremely tight.  I can't speak for hte M1S  combo as I gave it my dad but my complete Stag has loosened up a little bit after a 2000 rnds.

Quoted: I will give this a whirl when I get back.  The main reason that I don't like it being too tight is the fact that it prevents me from stripping it without tools of some sort.  It's kind of like needing a wrench for the full-length guide rod on your 1911

Understandable and a valid point but trust me, it will loosen up to a perfect fit in no time once you start using it. I have a couple of RRA and had to go through the same process and now they open\close and fit perfect

I was in your exact situation when I put my stag together, everything was extreemly tight.  After a few hundred rounds it will loosen itself up a bit.

Too tight is a good thing as long as they do in fact fit together without damaging force.  If you really don't like the fit, then you could try stoning the lugs on the upper a little bit, but this will loosen up with time anyway.  The more you disassemble it, the looser the fit will become.

My advice is just enjoy it as is.  All other things in order, your AR will be that much more accurate with a tight upper lower fit.

My RRA was extremely tight. It did loosen up after several hundred rounds. As every else pointed out tight is good. Give it time.