Gunsmithing AR15 barrel cant/ issues with barrel extension questions

[WildBill3/75]

Anyone ever seen this?

I have a mega arms ar15 monolithic....1st barrel I put on it was from a guy that does precision ar's for a living, lilja blank. When torqued down the barrel canted to the left, visibly. I sent him the upper/ barrel and he fixed it. Saying it was a faulty barrel extension and he tried with a few other barrels and they were good.

Flash forward to now I just got a new barrel from a known precision barrel manufacturer and same thing, barrel cant. I sent the upper back to mega where they looked it over and tried a few barrels and they say it's good. Today I even pulled a 5.56 upper apart that uses the seeking irmt -r. Same thing I'm getting barrel cant with this new barrel. I was measuring .040" cant from the handguards on the mega.

I'm going to send the barrel back to the mfg but has anyone ever seen or heard of this before? Or am I just this unlucky?

 

[jerseymike]

Not sure how common it is but I've ran into 2 barrels out of about 40 which had canted barrel extensions. One was a bcm the other a dpms.

 

[AKA-Spook]

I just had something very similar with a 6mm Grendle barrel I turned, canted hard left. Pulled that barrel and put a JN barrel in its place, same deal.
I put the upper receiver in the mill using the scope rail as a clamping point, just barely faced the receiver (.010) and reassembled .... It ended up perfectly centered in the scopes adjustment and appears to be inline with the receiver .

I am not sure how an extension could be built with a cant inducing surface? But I was wrong before...once

 

[bptactical]

Truing the face of the receiver ring is SOP on a "precision AR".
Another thing that can cause a cant is the barrel can rotate so ever slightly when tightening the barrel nut. Just a hair of play of the pin/slot can clock the sight to the left.
A touch of your preferred lube to the face of the barrel extension and holding the FSB or gasblock with a little "right english" while pulling the barrel nut to torque helps.
If the slot is really sloppy a shim can be fitted.

 

[AKONA]

If the receiver is true and the extension is true there is no canting possible. So it has to be one or the other or it could be both.
Ask anyone with a lathe to find the true center and see how much one or the other might be off. a thousand or so is ok. More then needs to trued.

 

[AKA-Spook]

Another thing that can cause a cant is the barrel can rotate so ever slightly when tightening the barrel nut. Just a hair of play of the pin/slot can clock the sight to the left.
A touch of your preferred lube to the face of the barrel extension and holding the FSB or gasblock with a little "right english" while pulling the barrel nut to torque helps.
If the slot is really sloppy a shim can be fitted.

....is this with regard to the axial alignment of the barrel / receiver union or the front sight leaning one direction or the other?

 

[bptactical]

....is this with regard to the axial alignment of the barrel / receiver union or the front sight leaning one direction or the other?

Canted/leaning front sight base, they will cant to the left when you tighten the barrel nut.
The barrel will skew if the receiver ring is not perpindicular to the axis of the reciever. That is pretty easy to see if you back away from the upper/barrel and look at it from the rear.

 

[ken226]

Canted/leaning front sight base, they will cant to the left when you tighten the barrel nut.
The barrel will skew if the receiver ring is not perpindicular to the axis of the reciever. That is pretty easy to see if you back away from the upper/barrel and look at it from the rear.

Just to make sure were talking about the same thing, when I hear that the barrel is "canted", do you mean that when your looking down the barrel, the front sight base is visibly leaning to left/right?

Or do you mean that the barrel is visibly pointed up/down/left/right and not centered relative to the handguard?

If the front sight is visibly leaning left/right, this has nothing at all to do with the extension. It means the front sight base taper pins were drilled without the front sight base properly indexed in a vertical position.

When the tenon is cut, threaded, chambered, headspaced and torqued to the correct value, the barrel is in some random position with no consideration taken for what is up/down/left/right. The gas port and front sight base are finished after the extension is installed and indexed on the extension.

To fix the problem can be really hard, or really easy depending on which route you want to go.

If you really really wanna stay with a taper pin FSB, then the extension needs to be removed by a competent machinist, the shoulder turned back on a lathe just the right amount that the extension can be installed with the proper torque value, and index with the taper pin FSB at the same time, then the chamber recut and headspaced.

The easy way would be to just remove the taper pin FSB and replace it with a pinch-bolt (clamp on) or set-screw style. The gas port may not be perfectly centered but this won't matter much because the FSB gas hole will be enough bigger than the barrel gas port that it will still work fine.

I recommend the second option, as the first would be much much more expensive.

If anyone here really wants it done using the first method, there are some really good machinists on this board who can do it, Including me!

If the receiver ring weren't perpendicular relative the the bolt track of the upper receiver, the barrel would be pointing up/down/left/right relative to the centerline of the bolt track or it would be parallel but not concentric, but very very likely it wouldn't be enough for the human eye to detect.

If it is the receiver ring and the barrel is pointing a different direction than its supposed to, facing the front of the receiver ring is a pretty simple procedure. I turned a piece of 6061 aluminum on centers to be an interference fit with the uppers bolt track, and with a 60degree countersink on each end. I just put the round rod piece inside the upper, put it between centers on the lathe and take enough metal off the front of the receiver ring to square it up. I then slide the receiver forward on the round rod fixture and indicate the inside of the receiver ring just to see how far out it is. Since the extension will index on the front face of the receiver ring, it won't matter much if the inside of the receiver ring is "out". There is usually enough clearance between the OD of the extension and the ID of the ring that it will easily true up against the ring face.

 

[bptactical]

Cant, cant·ed, cant·ing, cants
v.tr.
1. To set at an oblique angle; tilt.

I am in reference to a leaning ie "canted" front sight base/gas block.
If the barrel is not in plane with the receiver I would consider that "skewed".

If the front sight base was incorrectly aligned when the taper pins were drilled the bolt on base is a fix as is doing a "National Match" mod to the stock base. Remove the FSB. Open the taper pin holes up so you can tap them 8-32. When you drill the holes try to offset the holes slightly to the top. Tough to do with a drill but if you have access to a mill it is a breeze. Reinstall the FSB and align it correctly. Now mark the barrel where the new holes are. Remove the FSB and mill a small flat at your marks. Take small set screws and secure the FSB in place, aligning it vertically. It is a bit of work but for service rifle guys it is a good fix and allows fine tuning.
I have also done very well on fouled up FSBs by fixturing the barrel, indexing on the extension pin and using an oversize reamer and pins.

 

[WildBill3/75]

Easier with pictures, I was attempting to convey the barrel leans not the FSB being rotated.

 

[bptactical]

Easier with pictures, I was attempting to convey the barrel leans not the FSB being rotated.

True the face of the receiver ring.

 

[WildBill3/75]

True the face of the receiver ring.

Did you not read my post? I sent the upper back to mega and they looked at it and said it was good to go and I pulled the barrel on another upper and was getting the same deal. So I think the issue is the barrel/ barrel extension.

 

[Dtarget]

What is canted? The GB, gas port, FSB? It could be the way you are tightening the barrel nut.

 

[WildBill3/75]

What is canted? The GB, gas port, FSB? It could be the way you are tightening the barrel nut.

The barrel physically leans to the left side of the handguard. Same barrel does it in 2 different receivers. I just sent the barrel back the other day.

 

[bptactical]

I did read your posts amigo, I guess it just didn't click. Sorry for any aggravations.

 

[ken226]

The barrel physically leans to the left side of the handguard. Same barrel does it in 2 different receivers. I just sent the barrel back the other day.

Sounds like you're correct, the barrel tenon threads aren't cut right and the extension is threaded on a bit caddywampus, or possibly the extension is crooked.

If the tenon weren't cut right, it wouldn't be worth trying to fix the threads. The tenon needs to be cut off and a new tenon cut/threaded/chambered and headspaced. If that isn't an option due to the barrel length, it will need to be replaced.

It should be pretty easy for you to rule out the extension as the culprit. Mic the thickness of the shoulder and several points around its circumference, if its the same thickness, your good to go.

My guess is that the threads aren't cut strait.

 

[AnschutzNerd]

OP - I had the same thing happen with a Daniel Defense barrel. Faulty barrel. Fortunately, the vendor took it back.

Noveske barrels are my Go-To barrel after that experience.

 

[WildBill3/75]

Just got a phone call from Satern Custom Machining earlier today, they are the ones who made the barrel. It turns out the barrel extension was the source of the problem. Really happy with both Manufacture's customer service. Mega had my upper inspected and gave me a phone call the same day they received the package and mailed it back to me the next day. Satern also fixed the issue and gave me a call the same day they received the barrel and said they were packing it up to ship back. Couldn't ask for better customer service!

 

[bptactical]

And the crowd goes wild!
;-)