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Discussion Starter · #1 ·
So I built a milled fixed stock M70 recently, and I’ve noticed a few weird “dents” on the bolt and 1 related dent on the bolt carrier too. When I headspaced it, it might have been a bit closer to the loose side (no go) side, than extra tight, because I just didn’t want to continue to over tighten and then get too loose of a barrel fit, since my press is kind of crude and builds up pressure then often goes too far whichever way I needed it. So the 2nd time that I got the barrel in a position where it closes on a go, but not on a no go, I just left it, even though it was a bit closer to closing on a no go than would’ve been probably “just perfect”. It still didn’t close on a no-go and did close on a go gauge though.

After taking it out firing I double checked the headspace, and did so a few times. Something I tend to do usually for a few shooting sessions after finishing a new build, just to make sure everything stays consistent. While doing that I got to a point where I noticed that if I put a bit of force into it, I can get the carrier to slide all the way forward even on a no go gauge, though it requires some pressure and is a bit of a tough go (to clarify I’m not slamming it fast into the chamber, just putting some extra elbow into it when it starts hanging up). I re-checked with a field gauge after noticing that, and to my comfort I couldn’t force that one into closing. Mind you, by now I’ve shot probably close to 1000 if not more rounds out of it, 500 or so of which during the Kalashnicon event where it was the rifle I competed with. So I’ll include a few pictures of the bolt and carrier, and I’ve circled the areas of interest to me. Primarily the dents, but also the back of the lugs and such, to see what you guys think is going on, and if it’s a safety issue.

My assessment currently is that the back of the bolt lugs look fine to me, despite the dents I’ve noticed on other areas of the bolt. I didn’t lap the bolt or receiver locking lugs any when I built it, and it’s built on a tortort 80%. The lugs on the receiver as far as I can see also look fine. My thinking is that maybe somehow during “forcing it” to close on a no go, those dents were caused, and I’m optimistic still that the gun is still safe to go. What do you guys think based on the pics?
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Take the carrier out of the equation when checking headspace. Check it with just the bolt and see where you are at. To me it looks like something is not in spec and the bolt is being hammered back a bit.
 

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Discussion Starter · #3 ·
You know I kind of checked the initial headspace both ways, but ultimately use the one with bolt carrier as the standard seeing how I’m not sure at what point a little pivoting of the bolt is fine with no go and field, and when it’s too much. So I let the bolt carrier going all the way or not be the deciding factor
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Ok, how about these pics.

here’s 1 with cip go gauge

Here’s 1 with saami go gauge

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Here’s 2 with saami no go gauge

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And here’s 2 with I think Cip field gauge

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You know I kind of checked the initial headspace both ways, but ultimately use the one with bolt carrier as the standard seeing how I’m not sure at what point a little pivoting of the bolt is fine with no go and field, and when it’s too much. So I let the bolt carrier going all the way or not be the deciding factor View attachment 312349 Ok, how about these pics.

here’s 1 with cip go gauge

Here’s 1 with saami go gauge

View attachment 312350

Here’s 2 with saami no go gauge

View attachment 312351

View attachment 312352

And here’s 2 with I think Cip field gauge

View attachment 312353


View attachment 312354
Whats happening to your turning lug, I don't know. Maybe a foreign object got caught up in there and caused that. I say remove the burs with a file and keep watching it.

As for the rolled edge bur on the main locking lug (ejection port side) this is caused when the locking lug on the non ejection port side is not contacting the receiver lug at the same time as the main locking lug makes contact. What happens is when you fire the bolt is pushed back into the lugs, the main lug makes contact first and the other lug does not. This causes the bolt to tilt until that second lug does make contact. During this tilting action, the corner of the main lug on the bolt rubs the wall of the receiver and starts rolling that bur.
 

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Discussion Starter · #5 · (Edited)
Whats happening to your turning lug, I don't know. Maybe a foreign object got caught up in there and caused that. I say remove the burs with a file and keep watching it.

As for the rolled edge bur on the main locking lug (ejection port side) this is caused when the locking lug on the non ejection port side is not contacting the receiver lug at the same time as the main locking lug makes contact. What happens is when you fire the bolt is pushed back into the lugs, the main lug makes contact first and the other lug does not. This causes the bolt to tilt until that second lug does make contact. During this tilting action, the corner of the main lug on the bolt rubs the wall of the receiver and starts rolling that bur.
Hmm, so I probably should’ve tried lapping the bolt and/or receiver lugs then before I head-spaced the barrel to make sure everything was making even contact? Can’t say I ever lapped any before, so the idea of filing away however little, at lugs didn’t much appeal to me in case I screw it up somehow. So, I guess if that’s the cause (which I confirm by checking with some sort of lapping compound that one side is not contacting the way the other is) next move would be to see if I can lightly file either the main lug (the one on the ejection side) of the bolt or receiver (any preferences on which is better), to then get even contact on both lugs. And then I guess, most likely have to redo headspace? Or would it by now have potentially “evened” itself out after so many rounds, assuming it wasn’t a big gap in contact between the two lug sides.
 

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Hmm, so I probably should’ve tried lapping the bolt and/or receiver lugs then before I head-spaced the barrel to make sure everything was making even contact?
I'd do it the other way around. Press the barrel in until you have just enough headspace for the Go gauge. Then file the bolt's locking lugs to make sure they're even. Actually, you'll likely only have to file one of them, but you could have to file both if the tight one isn't making full contact with the surface.

As for what to do now, I think you should file the bolt's locking lugs so that they both make proper contact. Then check the headspace. If the bolt will lock on a No-Go gauge, you'll need to press the barrel in a bit and redrill the barrel pin for an oversized pin.
 

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First, find out what is actually happening. Take a spent case and cut it in half. Get a spring that just fits into it and have some of that spring hang out of the bullet end a 1/2" or so longer than a regular complete bullet's length. Strip your bolt and use a sharpy to color the back side of the looking lugs. Insert your spring cartridge into the chamber and then your bolt into the carrier and run it forward till it closes. Now pull the bolt carrier back and remove the bolt and see if any of the sharpy ink was scratched away from the lugs. This tells you if you have contact on both sides or just one side. Know whats going on before you start doing anything to remedy it.

Also clean everything before you do this test. You don't want any crud giving you a false reading.
 

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Something else to check. If you look at the drawings for AK receivers and trunnions, the lugs on them are not square with the bore on the barrel. They are basically like a screw. The more you turn the bolt, the further forward it goes. When checking headspace without the carrier you do the following:

Put the bolt in the carrier and bring the bolt to the locked position like it would be if locked into battery.
Mark the the bolt at the 12 o'clock position.
Remove the bolt from carrier.
Remove extractor
Insert gauge
Rotate just the bolt and bring the mark you previously made to the 12 o'clock position. That is where the headspace needs to be checked. If you over or under rotate the bolt, your headspce will not be correct.

Since the bolt and carrier have functioned without any issues for an unknown high round count before being demilled, I would suspect that something is out of spec between the receiver and carrier.. The carrier may have been hanging up or hitting the stop too soon to fully lock in the bolt. It could also be that the headspace was not set correctly.
 

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Discussion Starter · #10 ·
Still trying to figure things out with how to go about with the sharpie test. Don’t have quite the right spring and half of a cartridge, so been trying to get it done with a go gauge instead but not feeling like it’s going quite right. I guess while I still try to figure this out, anyone have suggestions on what kind of lapping compound would work good with any bolt lug fitting for potential future use?
 

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If you have a live round, pull the bullet out, dump the powder, put it in the rifle and fire the primer (it will be loud) and then you have a spent case. Now cut it in half with a dremel. Hardware store for the spring.

You need the straight backward pressure from the spring to make sure you get a good reading.
 

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Discussion Starter · #12 ·
If you have a live round, pull the bullet out, dump the powder, put it in the rifle and fire the primer (it will be loud) and then you have a spent case. Now cut it in half with a dremel. Hardware store for the spring.

You need the straight backward pressure from the spring to make sure you get a good reading.
Couldn’t get the same appropriate representation by chambering a dud round with the firing pin removed? I’m trying to figure out the significance of the spring that a dud cartridge doesn’t simulate accurately?
 

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OK, do you have rifle vice / cleaning stand? Put the rifle in that, put your go gauge in the chamber, close the bolt like normal, and use a cleaning rod or something down the barrel to push against the gauge as you pull the carrier back.
 

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Discussion Starter · #14 ·
Don’t have a rifle vice, but wound up improvising with a bench wise. Hopefully that works too. I did it once with the bolt on its own though just sitting on a chair, tilting it back and forth a few times to the position it should be in the carrier and got this

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Maybe I didn’t do it right somehow but it looks like scant removal of sharpie on both sides.

Then I tried another attempt pushing the cleaning rod on go gauge while cycling carrier back and forth a few times also on chair, and got this

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Then finally, tried with carrier with rifle in the bench vise same way,moving carrier back and forth a few times and again pushing go gauge w cleaning rod from muzzle, and got this

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Not sure what to make of the overall small amount of marker being removed regardless, assuming I did it right. With the bolt carrier involved it definitely seems like less contact seems to be made between the left (non ejection port) lugs. Not sure why though. And why so little marker was removed overall.
 

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From what I see in your picks, the ejection port side makes contact and the non ejection port side does not. Use a file, held at the 3* angle and remove material from the ejection port side bolt lug until the test shows contact on both sides. Take a swipe or two with the file and then recheck until contact happens.

Once that is done, if you want to, you can lap the lugs in to the receiver. Once that is done, press the barrel in till it stops on your bolt with the go gauge inserted into the barrel. Now re-pin the barrel.
 

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Discussion Starter · #17 ·
Was afraid it might involve re-headspacing but it beats having the gun blow up. Hopefully this time I’ll have it done right and avoid further headache. So yeah I’ll give that a try. Any suggestions on preferred lapping compound?
 

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Tooth paste will work but there are many grades of actual lapping compound.
 

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I recommend Pepsident for the tooth paste. It is gritty and does work for lapping things, It also works for filling in nail holes in sheetrock.
 
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