Hi all,

So I received the barrel and upper for my build, and of course tried to slide them together.  The fit gets so tight that I can't continue to slide the barrel all the way in.  It would need to be tapped in with a mallet. Is this normal?  What should I do about it?  Here are pics of the issue. Any help or advice is appreciated.

 

If it was me, i'd break out some 400 grit wet and dry and hand fit them. Use moly lube for assembly.

I was thinking that it may require some working into a proper fit.  I am thinking probably the barrel extension more so than the upper?

That's what I'd do. Check and make sure it isn't the pin that's getting stuck. I had one do that on me, a little filing on the slot in the upper fixed it.

Oh man... And I was going to smack the F*** out of it with a 12 pounder and then blame you for ruining my gun!
There goes that plan.  

Anyway,  It seems stiff enough that if I were to tap it into place, it would need to be tapped back out of place to remove.  I will inspect further and likely sand it a bit to relieve some of the tension and re-try.  

Really, the best way to beat your barrel in, is to attach a muzzle device and then hit it.






 

I've come across a few tight uppers.  I lap all of mine to ensure the face of the upper is perfect, and in the process of doing so it lightly laps the channel in the upper as well.  On a few I actually had to put a bit of compound on the shaft of the lapping tool and lap it into the upper.  Once there though everything was fine.  Makes for a nice slick operation as well, like a nicely fitted slide to an auto frame.

I have been working it back and forth a bit and it is kinda lapping itself without compound.  Only thing is i cant spin it past the guide pin.  Is there  a lapping tool that you would use for that?

That top picture looks like the barrel extension is in the upper crooked.

grease the dogshit out of both the barrel extension and the inside barrel tunnel of the upper, use a back & forth, twisting motion
when sliding the barrel extension in, paying close attention to the alignment pin alignment when it gets close
to being all the way in.

That fit looks ideal to me.
What you don’t want is for the BE or barrel socket in the upper to be tapered.

What he said. I'd prefer to have a tight fit than a loose fit so be very conservative when you lap that ID, .001 to .0015 should really be just enough to go.

Yeah the standard lapping tool would work for both.  On the main shaft that centers the tool within the upper you can put a scant amount of the compound on and work in into the upper.  Once it wil seat all the way against the face of the upper it can be lapped as well.  If you were anywhere near my part of the world I'd tell you to meet me and we'd knock it out in like 2 minutes.  Maybe you can find an arfy brother up there with a tool?  Otherwise they run about $35.

I would be reluctant to lap the socket as that would impact the protection the anodizing gives at a steel to alum joint over a relatively large area.

This, your anodizing will be more galvanic protection than your moly grease.  Most AR's assembled would be protected enough with just their anodizing in tact.  That can't be guaranteed always, so your moly grease protects and helps things slide into place.  

 

Last few posts, really good info guys, and thanks for the help.
I messed with it a bit more last night just working it back and forth and then finally it slipped into place and seated fully.  I was able to separate and replicate the fit so I think it is good to go.  I was just a little nervous about it at first.
Now to order the rest of the build and complete.

Glad you got it to fit!  

I had a Cerakoted upper that was tight fitting.  I put my barrel in the freezer for an hour and that made just enough difference that I could get it to go.

Had one barrel like that, but all I needed to do was tighten the barrel nut and it went in without any issues.  Removal might require lightly tapping the feed ramp end with a wooden dowel while the wife unit holds the barrel so it doesn't hit the floor.

Paladin

It's not uncommon to have this happen.  Most of the time this interference fit is a combination of slightly thicker than usual hardcoat inside the upper, a barrel extension on the plus-side of tolerance, and having it just slightly off axis while you're trying to insert it.  None of these things are bad, just a bit of a hassle.  

Use whatever grease you're going to use for your barrel nut, and put a light coat on the barrel extension.  Then (as noted above) screw on your muzzle device and start tapping on it.  Unless you're using an 8 pound sledge and you're The Hulk, a few taps should get it in fine.  You may eventually need to do some tweaking to exactly how rotationally centered the barrel is in the upper, so that your gas port is perfectly at 0º in relationship to the rail on the upper, but after you've tapped on the barrel a tiny bit, I think it'll probably just slide in.

DO use a quality grease on your barrel nut.  I use AeroShell 33MS because I like to stick to the TM's instructions (and now that I have a tube of it, I may as well, right?).  Stay away from cheap, automotive axle greases - the barrel nut isn't on an axle anyway, and you will find that most of them aren't really up to the standards of any quality synthetic grease.  A white lithium-based grease with 5% molybdenum disulfide is what you need.  Honda's Moly 60 paste has something like 10% MSO2, and it's high quality stuff.  But if you're going to build more than one or two rifles, spend a few bucks and get whatever grease you're going to use and set it aside for barrel work.  It's worth your time to do that.

Don't ever buy a Mur upper if you don't want to tap the barrel in.  Will not hurt to put the muzzle device on it and tap it with a rubber mallet.