Gunsmithing AR upper and barrel fit

[fredreded]

A friend and I just received our barrels from the smith to install in our MATEN upper monolithic combos (for precision shooting). I have put a few AR-15's together and the barrels always just slid right in by hand. When we went to put these on, they were VERY tight. We eventually got one to go far enough to catch the threads of the barrel nut with barrel assembly grease and some taps with a rubber mallet against brake (FTE). We then were able to use the nut to pull it in the rest of the way. That seemed excessive and took 45-60 minutes of working with it to get it together. The other one would not go. It only went in about 3/8ths of an inch then stopped cold.

Not knowing what to do, I went to the forums and found that this is not an uncommon problem. I called my smith (who built the barrel) and he said he had not run into it really either. I found that there are two ways people deal with this problem. Both seem to appear in equal numbers when someone asks for this kind of advice. The first involves heating the upper to 250 degrees or so and then they say it slips right in. Others say not to do this because it has the potential to mess with the temper of the metal and can crack the receiver. The second category of advice is to either use a lathe on the barrel nut, or use scotch brite to polish both the barrel nut and the inside of the receiver. Again, there are contradictory claims regarding this method. Some say don't use any abrasive at all because it can mess with the coating and leave bare aluminum which can apparently have the potential to corrode.

I am confused a little because I have read advice recommending one or the other from both professionals and amateurs. Likewise both professionals and amateurs advise against one or the other because of catastrophic failures.

I am not comfortable with the heating for three reasons. One, I would want to heat the entire upper to avoid uneven heat. This would make it difficult to handle. Two, I am afraid of the temper issue as well as cracking the receiver. Three, how would one get it apart without damaging it when I need to re-barrel? Mega themselves recommends this method. My guess is to keep people from taking a dremel to the upper.

As I think about it, I am more comfortable with the polishing with the scotch brite because I doubt I will take off much material and as long as I don't go through the coating on the receiver, I should do no harm. The goal is to polish just enough to get it in, not slop it up. Again, I am guessing Mega doesn't like this because of the potential to ruin the coating.

Full disclosure, I have already made my decision and put it together. I used the polish route. I held the scotch brite on the barrel and had my son turn it. I also made a mandrel and polished the inside of the receiver. It did not go through the coating (I can see no bright silver portions and all is still comfortably black). When I put it together, I could slide it in to the point of the indexing pin by hand (with assembly grease) and then with a good hard push, it seated down to the depth it was supposed to. It is still firm but I can take it in and out by hand now. This in my experience is more normal but I am a novice for the most part.

Now my questions are:

1.) When the two are not going together, which route should one use, polish or heat and why?
2.) What are the risks for each?
3.) In terms of accuracy, does it even matter at all?

A related question would be, given the different contraction and expansion rates of steel and aluminum, would one or the other be more accurate as the barrel heats up?

And lastly, I suppose I am looking to ease any anxiety I have lingering about fracking up my $1000 billet combo which incidentally I cannot replace because they don't make them any more... I shot my first groups while pressure testing in preparation for load development and the first several groups were outstanding. That helps ease the anxiety some!

My hope is to assemble a thread with good information for others in my shoes and have a definitive answer if I face this again. I am particularly interested in hearing from the veteran builders out there with lots of experience (calling GAP etc.!) Thanks for the input guys.

 

[fredreded]

A quick update with additional processing thoughts.

I got feedback on another forum that Gunkote requires a heat of 300 degrees to cure. That seems to settle the temper issue (as long as you heat it evenly anyway). That however does not solve the potential cracking issue on a barrel install where the tolerances are too tight.

I certainly see the potential problems with abrasives as well, particularly on the upper but as long as it is polishing and not removing much of the coating, I am less inclined to worry. The trouble of course is knowing when to stop. Perhaps I just got lucky.

Other knowledge or experience based input?

 

[turbo54]

The fact your able to get the barrel extension even started in the upper suggests to me you're dealing with a very slight interference fit. Remember, a 1.0000000" peg is a press fit in a 1.0000000" hole.

Your fear of cracking the upper would have merit if we were talking about a .020" press fit, but we're not. We're talking about a .001" (or maybe less) press fit.

Heat the upper to 150-200ºF and it'll go right in. You're not going to hurt the temper at all.

Because aluminum grows much more with temperature than steel or stainless, you'll be able to remove the barrel by putting the whole upper in the oven as well. Yes, the barrel extension will grow when heated (becoming larger in diameter), but the hole it fits into will grow *more*, allowing it to come right out.

 

[chakup]

Mine was the AR15 version, not the 10. But iirc the manual states to lube the outside of barrel/chamber where it slides into the upper. My WOA barrel required some taps to get it fully seated- just means Mega is holding tolerances tighter which was what the intent was going with quality parts correct?

 

[fredreded]

Ok. I think I am convinced. Heating in an oven to 200 degrees should be the first step to see if it goes in.

Now, on to the last question... Does it really matter? Or perhaps a better way of stating this question might be:

What effect does tight vs. loose barrel to upper fit have on accuracy? It seems to me that as long as it is within spec it should not matter. As long as the receiver is tight enough to align the barrel, and the barrel has been properly torqued in, the hold is accomplished by the barrel nut. Again, if it is within spec, I can't see how an extremely tight fit makes it more accurate than a reasonably snug fit.

Perhaps the issue is "tolerance stacking". That is trying to accomplish the reasonably snug fit across pieces and manufacturers means that the tolerances need to be on the high end for each piece, such that all combinations have at least a snug fit. This then results in some combinations being too tight and requiring extra measures for installation. However, precision manufacturers want to ensure every combination is snug because there may be accuracy degradation if the fit is too loose, resulting in improper alignment. Too tight on the other hand might not hurt anything as long as you can get it together.

Does this explanation seem reasonable (and more importantly) accurate?

 

[turbo54]

It is generally believed that a snug fit helps accuracy. You won't find many people that think a nice wiggle between barrel and barrel bushing on a 1911 is good for accuracy.

How much does it matter? Not sure. I can say my DPMS LR308 seemed to go from a 1 moa gun to a 3/4 moa gun when I shimmed my barrel to the receiver (a couple wraps of aluminum foil).

Manufacturing tolerances are definitley a factor too. It is nearly impossible to spec a hole at 1.2500" +/- .0001" (that is one tenth of one thousandth of an inch), because it is difficult to achieve such precision, adds a ton of cost, and rarely even matters to begin with - for most applications. Obviously, ball bearings have to be high precision.

So, when you design something for manufacture, you consider what degree of precision you need for the part to work as desired, and then you consider your manufacturing process. As you suggested, you'll run into some part combinations where you've got a minimum-allowable sized hole, and a maximum-allowable size peg. We call that "maximum material condition", and it provides the least clearnance available.

If you are interested, read up on Geometrical dimensioning and tolerancing (GD&T), according to ANSI-Y-1982-14.1

 

[geraldsd]

I have recently built two MATEN's. I found the uppers and the barrel extensions to fit very tight in both rifles just like you mentioned. I did not use an oven to heat the receivers. I pulled out my 320 grit sandpaper and removed the finish for the inside of the receiver and was able to work the extension into the upper receiver. I used a little lubricant to help. I agree with using a rubber mallet to LIGHTLY persuade the barrel to move as well, but be sure and protect the crown of the barrel. I really appreciated the tight fit. Both rifles are .308 and are capable of very fine accuracy. I, too, am happy that Mega is sticking to the proper tolerance as well. I also used Badger barrel extensions for my barrels.

 

[BigBlue&Goldie]

I have even had success freezing barrels. While you don't get as much thermal contraction, sometimes it's enough to allow an extension to slip right in. Hell, I even froze a set of balljoints before I pressed them in the other day.

 

[Beef_Supreme]

Freezing the barrel AND the upper works well, and does a nice job. After assembling high end ARs for several years with tighter than normal tolerance on the upper/barrel extension mating surfaces, I used a freezer on more than a bunch of occasions.
On the flip side of the coin, the old technique of bedding the two surfaces together with some blue loctite (applied correctly) works out VERY well and can really make the difference between a shooter and a non shooter.
I finally got around to working on a Blackout for ME the other day and I stuffed the barrel in a VLTOR VIS II mono upper and it was a bit of a struggle. Thankfully talking to it properly really made a difference and it all came together.
Good luck!