Gunsmithing AR-15 Barrel Extension Clearance

[jim10801]

I have a Rock River Arms NM A4 20 Inch CMP Rifle (2016) LAR-15, which is used in NRA High Power and CMP shooting. The current original barrel is showing some throat erosion. I purchased a 20” HBAR replacement barrel from Krieger that will be installed down the road.

Did some research and purchased what was thought to be all the tools and parts needed. That was before becoming aware of the bedding of the barrel extension. Have read about the Loctite method, the shimming method, and the Thermo-fit (heat shrink) method.

I have two upper receivers, the receiver in the gun and a brand new spare from RRA.

The Krieger barrel extension has an outside diameter measurement of 0.998”. The new spare receiver has an inside diameter measurement of 1.002”. These were carefully measured and are accurate to within a tenth or two. That would give me a clearance of .002”. The big assumption is that the current receiver in the gun has a similar ID. If it is much larger I will consider using the new spare.

A lapping tool is coming from Brownells for truing the face of the receiver. At this point that is all I intend to do as far as “bedding” goes. Thinking in terms of installing the barrel without Loctite or shims. Will see how it shoots without the bedding and then decide if and how to bed it.

My question is this. Is the .002 clearance considered reasonable in the world of barrel extension fit-up? Is bedding even advised?

TIA

 

[camocorvette]

If you can buy .001 shim stock I say put one in and loctite.
I'm not a Smith nor have the knowledge of one. I think some one like @LongRifles Inc. might have an answer.
Im tagging a video. I suggest watching all of his (like 6) but watch the one posted. That might help you with you path of decisions on assembly. Hes got a wealth of knowledge on the platform.
As for the professional answer I'm sure someone with much more knowledge will pass through and answer.

 

[Wheres-Waldo]

Seeing as how an A3 stripped upper is $60 or so, I rock set them in. When the barrel is toast, the upper receiver goes away with it.

Just make sure to clean up any excess around the ass end of the extension inside the receiver, as well as any that squeezes out around the shoulder before installing your barrel nut.

 

[Greg Langelius *]

I'm in unison regarding treating the stripped upper as being a consumable on just about the same order as a barrel. As consumables go, it has a very reasonable replacement cost. As for the labor involved, that's almost laughably simple; and something any AR owner should be prepared to perform as needed.

When I've assembled AR's, my most recent was a 6.5G, lapped and bedded with Lok-Tite Red. I need more time to evaluate results, and my real test bed will be another 6.5G with a spare barrel. It came as a factory assembled complete Upper, and has none of these embellishments. Once lockdowns recede and load development is completed, I have an identical barrel which will be installed following the Lap and Bed process. The accuracy will be compared. I have nothing but hunches to go by regarding the procedures' efficacy and; I don't like basing recommendations on hunches.

I'm going to develop the facts. If the facts correlate with my hunches, I've got about a half dozen AR's that will need a retrofit with Lap and Bed.

FWIW, I draw a distinction between the terms "build" and "assemble". If there's no genuine machining involved, it's an assembly. I do no machining and I am not therefore anything like resembling a Gunsmith.

I'm also highly impressed with the American Gunsmith Joe Carlos Video(s), and I have bookmarked them for future reference.

Greg

 

[jim10801]

Thanks for the responses. I viewed the videos by Joe Carlos. I contacted him with additional questions. He convinced me that the lapping tool sold by Brownell's and similar products sold by others was not the best way to go when truing up the receiver face. Further research on the internet convinced me to use a lathe with the upper receiver properly mounted in the lathe.

One of the issues with the lapping tool is the shaft that extends the length of the receiver and requires lubrication because it rotates while acting as a guide is that the clearance needed to allow rotation can also cause the lapping face to cant and not be square to the axis of the bore of the receiver, thereby defeating its purpose.

As mentioned, a fair amount of time was spent researching the web and reflecting on the best way to secure the receiver in the lathe so it is square. It was decided to use an expanding mandrel and this concept was tested with a 7 3/4" long piece of 6061 T6 1" aluminum pipe. The 6061 T6 material is the same that the RRA receiver is made of and the dimensions are close.

I used a dial indicator to get it all set up and took some test cuts:

Once I was happy with that I mounted the receiver itself:

And completed the facing cut:

This is a spare receiver. The gun that is going to be rebarreled will be disassembled later in the year and its receiver will receive the same treatment.
The barrel will be bedded with a shim and 620 green Loctite. Hoping it will be a good shooter.

 

[81STFACP]

Speaking of Joe Carlos' work, what do y'all think about his bolt carrier anti-tilt mod?

 

[jim10801]

Response to a PM from Sean:

The expanding mandrel and the 5c collet block set was purchased from Shars Tools. I think the 5c collet set came from Grizzly Tools but my memory is a little fuzzy.

I'm a believer in "Buy quality tools and cry once or buy cheap and cry every time you use them". Having said that, manufacturing standards from Taiwan have steadily gotten better over the past several decades. You can get some very nice stuff if you are selective.

Hope that helps.

 

[GONE BAD]

So let me ask this:
Why not have whoever builds your barrel install a barrel extension that thermal fits to your receiver along with a bolt that is headspaced?
I mean were talking accuracy here aren't we?
I've screwed around with the glue thing and yes it kinda worked but the last barrel i got had to be thermal fitted and I guess I got lucky, it is probably the most accurate ar I have.

I will also add, no receiver lapping, no blue or red shit running all over.

Now as for rebarreling this receiver IDK but time will tell but it seems to me that some here treat the receiver as expendable so no biggie I guess.

Just throwing this out there

 

[smoothy8500]

White Oak barrels and BCM Upper are thermally fit. For $60, yeah it's an expendable part. Regardless, I have to disassemble to retrieve the FF tube on my NM AR anyways.

 

[iktomi]

Response to a PM from Sean:

The expanding mandrel and the 5c collet block set was purchased from Shars Tools. I think the 5c collet set came from Grizzly Tools but my memory is a little fuzzy.

View attachment 7664688View attachment 7664689View attachment 7664690

I'm a believer in "Buy quality tools and cry once or buy cheap and cry every time you use them". Having said that, manufacturing standards from Taiwan have steadily gotten better over the past several decades. You can get some very nice stuff if you are selective.

Hope that helps.

Thanks.

I was looking at those parts specifically earlier today.

I appreciate the confirmation.

 

[gilk]

Thanks for the responses. I viewed the videos by Joe Carlos. I contacted him with additional questions. He convinced me that the lapping tool sold by Brownell's and similar products sold by others was not the best way to go when truing up the receiver face. Further research on the internet convinced me to use a lathe with the upper receiver properly mounted in the lathe.

One of the issues with the lapping tool is the shaft that extends the length of the receiver and requires lubrication because it rotates while acting as a guide is that the clearance needed to allow rotation can also cause the lapping face to cant and not be square to the axis of the bore of the receiver, thereby defeating its purpose.

As mentioned, a fair amount of time was spent researching the web and reflecting on the best way to secure the receiver in the lathe so it is square. It was decided to use an expanding mandrel and this concept was tested with a 7 3/4" long piece of 6061 T6 1" aluminum pipe. The 6061 T6 material is the same that the RRA receiver is made of and the dimensions are close.

View attachment 7403323

I used a dial indicator to get it all set up and took some test cuts:

View attachment 7403324

Once I was happy with that I mounted the receiver itself:

View attachment 7403325

And completed the facing cut:

View attachment 7403326

This is a spare receiver. The gun that is going to be rebarreled will be disassembled later in the year and its receiver will receive the same treatment.
The barrel will be bedded with a shim and 620 green Loctite. Hoping it will be a good shooter.

A few questions:

1. Why use a collet if it will be held in a 4 jaw chuck? Why not grab the mandrel with a the chuck - the flats should drive it - or use a dead center and dog?

2. What is the detailed dialing in process and what does your dial indicator setup on the inside of the reciever look like?

3. What level of concentricity are you getting between the receiver bore measured at the extreme ends and how are you measuring it?

4. How does the level of alignment/concentricity achieved with this method compare to others? What other have you tired and what results did you achieve with those?