I just put together an upper and I have a quick question.  When I have the barrel installed on the upper, it appears that the chamber doesn't fit flush with the receiver.  There is a small lip created by the inside of the extension not lining up completely with the inside of the receiver.  I checked and the barrel was fitted completely into the receiver.  The extension was flush with the threaded portion of the receiver when I tightened down the barrel nut. Does the inside of the extension have to fit flush with the inside of the receiver?  Did I install this barrel wrong?  


This is basically what the inside looks like.

The face/end of the extension generally is set back a little from being flush with the receiver. I guess it might could cause a problem if the barrel stuck out past the inside of the upper but I am not sure it would even then.

Nope, nothing wrong.

That seems to be an unimportant dimension.  Your magazine will clear it, and the bolt carrier does not contact it.  

The drawings I found on the net imply +-0.005.

Does your upper chamber a round and lock up?

I am still waiting on my bolt/carrier to test fit.  The extension doesn't stick out past the upper it's set in away from the receiver.  It's a very small amount and from what I gather it shouldn't cause a problem.  I am getting my bolt/carrier next week and I'll check for test fit when I get it.  

Stupid manufacture tolerances of either the upper or the extension depth, most likely the maker of the upper.  Should really be of no concern, as the bolt locks inside the extension.  

With the upper seperated from  the lower, drop a round into the chamber, and close the bolt assembly, and look at the rear of the carrier..... if it is reasonably flush with the upper,  carry on.

Quoted: Stupid manufacture tolerances of either the upper or the extension depth,...

AGREED!

I have not had this problem with an AR upper, but I have not worked with many yet.

EVERY STINKING SPRINGFIELD ARMORY M1A receiver I built did this, as well as an Entreprise Arms M14S receiver.  I went through SEVEN (SA) and ONE Entreprise Arms to get FOUR that I could MAKE work.  I had to machine and fit the barrel to the receiver.  It should NOT be this way.  A USGI or Kreiger barrel should screw-in and index on the mark, without having to lap the bolt lugs, or cut the barrel shoulder, or shorten the receiver ring.  I was only able to do it with four: that is 50% saved from 100% COMPLETELY made WRONG.  Poor quality has no excuse.  Of the three USGI M14NM receivers I worked with, when we had our State rifles, I never had a problem.  USGI and Kreiger barrels screwed-in and indexed on the mark.  USGI M14NM receivers were top notch quality.  I think the main reason it is so hard to get a great shooting M1A is due to the lack of quality receivers.  USGI M14 parts never gave me a problem, but stear clear of those cast/machined aftermarket BS parts, unless you want to make a rifle whose parts do not interchange.

Why is this a problem?   Only in the critical eye of the builder in this case.  This measurement could probably be off +-1/16" with noone noticing a function problem.

If this was a serious problem, the board would be full of chatter about out of spec uppers.  THIS DOES NOT MATTER.  

It is not an M14 reciever, an out of spec cam, nor mismatched locking lug surfaces, nor that firing pin slot in the M1A bridge.  

Some dimensions are critical in an AR, while others are not.  Fortunately very few dimensions are critical.

Quoted: Some dimensions are critical in an AR, while others are not.  Fortunately very few dimensions are critical.

This is one of the great things about the AR design.