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Mysterious new grooves formed on barrel chamber face

This is a discussion on Mysterious new grooves formed on barrel chamber face within the The M14 forums, part of the M14 M1A Forum category; Hello all! I seriously baffled here. Yesterday I received my new Gas Cylinder, (Thanks to Treeline) so I shimmed it (Thanks Jake2far) and installed it. ...


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Old April 7th, 2013, 07:24 PM   #1
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Mysterious new grooves formed on barrel chamber face

Hello all!
I seriously baffled here. Yesterday I received my new Gas Cylinder, (Thanks to Treeline) so I shimmed it (Thanks Jake2far) and installed it. I also installed a new Op Rod Spring because the old one was about 1.5" short. This morning I took with me my reloaded Nosler 168 Hollow Point rounds to the range.

Now before I start, my rifle has over 1500 rounds through it. I've been feeding my rifle primarily PMC 147g Bronze ammo and NATO Surplus Portuguese ammo. I just recently started reloading and so far I've shot about 60 rounds of 168 Nosler hollow points previously. Never shot hollow points in the past.

I set everything up at the bench and loaded my first magazine with 5 rounds of Nosler Hollow Point rounds. I released the bolt with the SEI bolt release I installed and I get this "Click". So I look at the op rod and it moved only 0.5". I looked in the chamber and found the tip of the first round was stuck perfectly just below the feed ramp. so I locked the bolt back, removed the magazine, inspected the round, loaded it and then reloaded the rifle by pulling back on the op rod as far as I could and then releasing. It loaded the same first round just fine. I shot all five rounds with no issues at all. I took the same magazine to repeat the process but the same exact thing happened! So I thought, ok let's try a different magazine. loaded 5 Hollow point rounds and the same exact thing happened again! So I took a closer look at what was going on and found this:



Those 2 grooves have gone unnoticed till now and I would have never noticed if it weren't for the hollow points. The tip of the hollow points were getting caught there on the first round. Now the funny thing is I shot 30 rounds of Hornady A-Max 168g red tipped rounds later today and they fed perfectly and never even touched that grove. They just glided into the feed ramps into the chamber.

Here's a view from the magwell


The markings are consistent with the bolt face and the rotation it makes when it locks/unlocks from battery.

I'm surprised I'm discovering this now because it's barely noticeable till you really look in there. The barrel is Stainless Steel and I understand it's a softer metal and after 1500 rounds, has anyone experienced this issue where the bolt marks the face of the barrel? It seems that hollow points may no longer be an option for me.

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Old April 7th, 2013, 07:40 PM   #2
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May be a minor clearance issue on the tapered counterbore. Due to variences in commercial receivers and barrel and the fitting involved it might have been a tolerance stacking situation.

If the receiver thread timing was off a little requiring more to be turned off the barrel torque shoulder which moved the c-bore closer .Usually if everthing is correct the bolt locking lugs front surfaces will hit the receiver ring before the front of bolt impacts the c-bore.

Sometimes this situation will cause extractor to bind /impact and will even disengage .

Thanks from budster and yellowthunder
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Old April 7th, 2013, 07:41 PM   #3
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Is that a crack at 10 o clock?

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Old April 7th, 2013, 07:46 PM   #4
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Is that a crack at 10 o clock?
Great pics. I was thinking the same.

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Old April 7th, 2013, 07:52 PM   #5
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No tis just a scratch.

Thanks Iron Worker, that makes a lot of sense. I only had 0.003" removed from the barrel shoulder to time the barrel just right.

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Old April 7th, 2013, 08:02 PM   #6
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No tis just a scratch.
Not the best lighting, but that line appears to be straight, and go all the way back on the shoulder. Not saying you are mistaken, but personally I prefer to err on the side of caution when there's a contained explosion inches from my face.

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Old April 7th, 2013, 08:15 PM   #7
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Pull the op-rod handle fully too the rear and let it fly. The magazine spring maybe stiff enough that there is some added pressure on the feed lips and the shorter distance from the bolt release may not have enough energy to strip the top round off and feed the beast.

Just like a 1911, sometimes there is enough slide energy/momentum using the slide release sometimes not even with new springs.

Thanks from nf1e and yellowthunder
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Old April 7th, 2013, 08:38 PM   #8
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Not the best lighting, but that line appears to be straight, and go all the way back on the shoulder. Not saying you are mistaken, but personally I prefer to err on the side of caution when there's a contained explosion inches from my face.
Dire88 Thanks,
I just inspected the site again with a flashlight and inspection mirror and it's definitely a scratch. It doesn't even make it halfway to the shoulder. Thanks for looking out for me!


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Old April 7th, 2013, 08:41 PM   #9
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Pull the op-rod handle fully too the rear and let it fly. The magazine spring maybe stiff enough that there is some added pressure on the feed lips and the shorter distance from the bolt release may not have enough energy to strip the top round off and feed the beast.

Just like a 1911, sometimes there is enough slide energy/momentum using the slide release sometimes not even with new springs.
Absolutely, Thanks Phil I'm going to make that a habit for now on. I put that SEI bolt release to do that cool AR-15 slap while smoking a cigar but it's just not practical.

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Old April 7th, 2013, 08:56 PM   #10
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GREAT pictures!!!!

This is EXACTLY one of the things that can/will happen when the barrel shank protrudes too far back from the receiver ring. The other thing that can happen is the front of the left bolt lug wil impact on the barrel shank when the barrel sticks too far to the rear from the receiver ring. In either or both cases the fix is to put the barrel in a lathe and cut shoulder and or shank a little shorter.

(Jersey Devil, have you seen these pics? Something like this could happen with your LRB receiver and GI barrel.)

Thanks from yellowthunder
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Old April 7th, 2013, 09:11 PM   #11
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GREAT pictures!!!!

This is EXACTLY one of the things that can/will happen when the barrel shank protrudes too far back from the receiver ring. The other thing that can happen is the front of the left bolt lug wil impact on the barrel shank when the barrel sticks too far to the rear from the receiver ring. In either or both cases the fix is to put the barrel in a lathe and cut shoulder and or shank a little shorter.

(Jersey Devil, have you seen these pics? Something like this could happen with your LRB receiver and GI barrel.)
Awesome Gus!
So when you say when the "barrel shank protrudes to far back from the receiver ring" do you mean that the barrel face is recessed so to speak vice protruding from the receiver ring? The barrel shank is pretty much flush with the receiver ring. The barrel shank does protrude I'm estimating about 0.005 with my calibrated eye ball and tactile Mk1 Index finger. There's no evidence of left lug impact thank goodness.

When I had installed this barrel, I had a professional gunsmith take off only 0.003 off the shoulder so that I could hand time the barrel to the receiver ring. That's all I had removed. Could it be a manufacturing error and is this something I really need to address?

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Old April 7th, 2013, 11:15 PM   #12
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Hey Dom, good to talk to you tonight. We need to make a point of talking more often. Was kind of tough when you were off doing your secret navy seal/ranger/delta force missions over in the stan. I know you are a spook and just can't admit it lol. As you know, all the KBI bbls need to have the shoulder trimmed back, we had that convo years ago, it is not unusual so don't think for a minute that you did the wrong thing. I have a few builds using these bbls as well. SO far I have only had one that hand indexed where I did not have to have anything removed. Try this on for size, KBI makes DMR med weight bbls for the military. The bbls we have are set up to index at 20-22 degrees. The dmr bbls are set up to 33-35 degrees. I have no clue why the military wants that much on the shoulders of the bbls. I only know this because of that receiver that I had the lug welded on and was possibly needing a custom bbl. One of these was suggested.
As I said I will send out a couple of emails and see what is suggested for a fix. I don't think your bbl is protruding too much because I have a couple that stick out further than yours. not to mention, you can move the bolt for and aft a little. I am wondering if the bbl is soft but it will have to go back to kbi for testing and possible machine work. I will give you a call later this week. You built a good stick! Its not your skills!! We will get to the bottom of this.

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Old April 8th, 2013, 07:33 AM   #13
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Called KBI bbls, sending pm

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Old April 8th, 2013, 07:46 AM   #14
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Several thing to consider and easy to check:

With the op rod out and bolt closed in battery there should be some for and aft play in the bolt .005" - .008" at least.

The receiver ring thickness .701" -.003"
Barrel thread (shank length) .695" -.005

If both are correct .003" - .008" shorter on the shank than the receiver ring. If the barrel protrudes past the back of the ring one or both are not to spec.

The bolt pilot o.d. length is very close to same depth of the barrel counterbore depth , if you have a incorrect torque shoulder timing issue or receiver timing issue plus inccorrect barrel all these thing add up to interferance at this location. I've seen this quite a few times ,at one time I had quite a few demilled barrel receivers USGI and many of them had impacted chamber faces.So not uncommon.

Bottom line is if the barrel is sticking through receiver its not correct,forward bolt lug face should contact receiver before bolt pilot hits chamber . Easy fix if you pull the barrel, simple skim a few thou off tapered face where the contact is.

Realisticly a cratex bob on a dremmel will polish that out and save yourself the headache.

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Old April 8th, 2013, 07:59 AM   #15
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Awesome Gus!
So when you say when the "barrel shank protrudes to far back from the receiver ring" do you mean that the barrel face is recessed so to speak vice protruding from the receiver ring? The barrel shank is pretty much flush with the receiver ring. The barrel shank does protrude I'm estimating about 0.005 with my calibrated eye ball and tactile Mk1 Index finger. There's no evidence of left lug impact thank goodness.

When I had installed this barrel, I had a professional gunsmith take off only 0.003 off the shoulder so that I could hand time the barrel to the receiver ring. That's all I had removed. Could it be a manufacturing error and is this something I really need to address?
Before I go further, I want to make sure you understand that cutting .003" off the barrel shoulder to get the barrel to draw up is not a problem as long as there was still good resistance when you aligned the barrel. Having to cut some off commercial barrel shoulders was something we occasionally had to do even with REAL M14 receivers. We never had to cut the barrel shoulders with chrome lined barrels or the old NM LIGHT or standard contour NM barrels, but we did have to cut some barrel shoulders with some Medium Heavy and Heavy NM barrels. Commercial receivers are not made to the same tolerances as GI receivers, so we have to cut the barrel shoulders more often with them than we did with GI receivers. It is just something that occasionally has to be done.

As far as how far back the barrel shank is supposed to protrude behind the receiver ring, the mil spec standard is the shank does not protude hardly any (only a few thousandths of an inch) or not at all on real US GI M14's and M1 Garands. For example, I have an early 520,000 series M1 Garand receiver and the barrel shank only protrudes maybe .002-.003" near the top of the shank, if that, and that's with a much later 1954 barrel installed in it. Jersey Devil has also confirmed this with the 10 REAL USGI M14 rifles his agency has on hand.

Now, what are the reasons for minimal or no protrusion of the barrel shank? They are:

1. So the barrel shank does not keep the bolt from camming in and out. I have never seen this happen with a USGI M1 or M14 receiver and of course when the barrel shank is made to spec. However, commercial receivers are not held to as tight of tolerances, so a barrel shank that IS made to spec, may cause this in a commercial receiver. What happens when the barrel shank is too long is the bolt can hit the rear of the barrel shank on the front of normally the left bolt lug and can keep the bolt body from camming down or the front of the bolt lug rubs hard against the barrel shank.

Now this is not to be confused with the bolt not camming down freely in a BARE receiver with no barrel in it. When that happens, there is not enough room from the front of the receiver lugs to the rear of the barrel ring. BUT, this is not the case with your rifle.

2. Sometimes when the bolt does cam in and out freely with the barrel installed and the barrel shank is too long for that receiver, the front of the bolt winds up hitting the shoulder inside the chamber as it is doing on your rifle and causes that impact groove. The fix is to pull the barrel and have that shoulder cut forward on a lathe until that groove is just barely gone - as other members have already mentioned.

-------------------------------------------------------------------------------------

In normal operation, the bolt speed is slowed down by stripping and pushing a cartridge into the chamber and even a little bit by the FP tail hitting the receiver bridge. So if the barrel shank does protrude a little, the bolt doesn't hit it that hard.

Where the bolt is going to hit hardest against the rear of the receiver barrel ring or barrel shank is when there are no rounds or magazine used that slows down the bolt. IOW, when the bolt is locked to the rear and allowed to go home when there is no round or magazine in the rifle. When this is done enough, that is usually when any impact on the barrel shank or inside the shank on the barrel shoulder will happen. Now I am not saying the bolt should not be locked back and released that way as it was/is a very common thing and the receiver and barrel are made and supposed to be fitted so it is not a problem.

I can't address whether or not the KBI barrel is "too soft" as I don't have extensive experience with these barrels. However, barrels are not hardened, but rather the strength of the barrel comes from the alloy of steel the barrel is made from. I don't think the barrel steel is too soft because you have already fired a bunch of rounds through it. Rather, I think the "geometry" of how the parts fit is what "is going on" in your rifle and your barrel shoulder just needs to be cut forward a little.

Thanks from budster and yellowthunder
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