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MG3 Barrels, Bolts and Headspace

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Waffentag
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MG3 Barrels, Bolts and Headspace

Postby Waffentag » Wed Dec 13, 2017 8:32 am

Well, I found another new surplus BW chrome lined MG3 7.62 barrel for $100 at a gun show. They seem to be out there. I also got a good deal on 4 Yugo bolts. That makes 5 total 7.62 barrels and 12 bolts, 2 WW2 (that I will not shoot), 0ne MG3 Bolt and the rest are M53 bolts.

I noticed that the BW Chrome lined barrels seem tighter than the unlined barrels. I am using the Gage set from Brownell Forster for 7.62 NATO.

So, I have two barrels that head space with just about all of my bolts. I have one barrel (BW 7.62 chrome lined) that is too tight and will and not head space with any bolt. I have one bolt head that is too tight and will not head space with any barrel. Then I have two barrels that headspace with only a couple of my bolt heads, the bolts lock up with the go gauge. I get full lock up, but you notice that the rollers snap in a little tighter than with the other barrels. Factory ammo seems to lock up fine.

So, I do not think that I will ever find a bolt head that head spaces in that one BW 7.62 chrome lined barrel with short headspace. Does anyone have an overly loose bolt head that they want to trade?

If not, is my best option to "remove the barrel extension and add a shim between the barrel shoulder and the extension. (for too short of chamber situation)." If so, where can I get the shims, and what do I need to tell my gunsmith to watch out for?

Also, is this a good option for my two barrels that gage, but are just a tad tight? Or will shooting them cause the headspace to loosen up a bit over time?

Thanks in advance for the advice,

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Re: MG3 Barrels, Bolts and Headspace

Postby JBaum » Wed Dec 13, 2017 10:08 am

Headspace might loosen with wear, but it would take a few thousand rounds. I'd probably try a steel bore brush on a rod mounted in an electric drill, with some valve grinding compound on the brush to see if a few thousandths could be taken out of the chamber. The success of that will depend on where the tight spot is. If it's fully forward in the chamber (where the bullet goes into the shell), it might be difficult to fix. Using a chamber reamer on the chrome lining won't be easy, and might chip out the chrome so I wouldn't recommend that.

The overall length of the chamber could be short, or the chrome could have reduced the chamber diameter dimensions too much at one place or another. Do a little research to see exactly what the problem is, then proceed from there.
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Re: MG3 Barrels, Bolts and Headspace

Postby Waffentag » Wed Dec 13, 2017 12:56 pm

John, On the rollers, I am unclear on what you mean. I though that before when I had asked this question, you said that it would not work because the rollers don't set head space, its the roller pins? Did something change in your understanding of the impact that MG42 rollers have on headspace?

I am not clear on the difference between the rollers and roller pins that you mentioned in an earlier post, So if I put the rollers in an electric drill (the chuck of a small drill press) and using the valve grinding compound on them, where do I need to grind? The portions that sticks out on the top and bottom of the bolt? Is this portion what you are calling the roller pins? Or do I grind the part the sticks out on the sides?

viewtopic.php?t=11957

Waffentag » Thu Sep 28, 2017 11:15 am
"Will replacing the rollers on an MG42 change the headspace?
I was able to tighten up the head space on my PTR91/G3 by swapping out the rollers. would this work for the MG42? But, they make rollers in different sizes for the G3.

Re: Replacing locking rollers"

Quote
Postby JBaum » Thu Sep 28, 2017 4:55 pm

"Different bolt/roller design. The rollers themselves don't set the headspace on a 42, it is the roller pins. With a 42 bolt, the roller pins can't be a different diameter, because they wouldn't fit right in the bolt head if they were."

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Re: MG3 Barrels, Bolts and Headspace

Postby JBaum » Wed Dec 13, 2017 1:59 pm

It would seem that what I posted in my first reply was a disaster - caused by staying up until 4am translating German (best excuse I got), and not being fully functional when I first got up this morning (at 10am).

My suggestion works well enough for HK stuff, but not for the 42. Sorry. I'm not even going to elaborate on what I had in mind for the 42 rollers, because I wouldn't want anyone to try it. I've deleted the poorly worded reference from the previous message I posted, so I'll try this again:

Far better would be to take the barrel out of the bolt head and either use some shim stock to keep the barrel from screwing in so far, or even run a small bead of weld around the barrel extension surface where it contacts the barrel, then cut it back down (lathe) it back down until the barrel headspaces correctly when bottomed out against the extension. Also possible would be that a different barrel extension would solve the problem, so maybe you could solve the problem with a different extension from RTG, or even an extension from a drilled/demilled barrel if that's cheaper. It's my guess the factory can't make every part perfect, so they would try different barrels and extensions until they got it within specs and then stake that combination and pass it down the line.

Is the face of the bolt contacting the barrel face, or does the cartridge not fit into the chamber far enough for the rollers to go fully outward?
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Re: MG3 Barrels, Bolts and Headspace

Postby Waffentag » Thu Dec 14, 2017 8:26 am

John, Thanks, Your last post got me thinking about that the problem may be with the barrel extension. This a new old stock BW Chrome lined barrel, looks like it was never fired. The barrel extension had some sharp edges from being so new.

So, I took my loosest bolt head and wedge, my 7.62 gages, a German 7.62 Live Round. The bolt would lock up nicely on the German made 7.62 Round, but not on the Forster Min or Max gages. I tried the German 7.62 round in my Dillon 308 case gage and it gaged at the max length for 308. This indicated that the head space must be close.

Looking at the channel for the rollers in the barrel extension , I noticed that the back of the channel that the rollers rest on is flat. So, I took a small file to remove any burs and took off about a thousands of an inch off the flat back of the roller channel.

With the one Bolt head, I was able to get good bolt lock up with the Go/Min Gauge. As it should be, the no go gage would not go.

With some of the other bolt heads, I was able to get them to lock up with a small bit of thumb pressure on the wedge. They would snap in with a soft click. In my opinion, they are still a bit too tight to use.

So, I will try the one bolt that gets good lock up with Germany factory ammo.

Do I have a solution to this headspace problem? Am I missing something with my solution?

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Re: MG3 Barrels, Bolts and Headspace

Postby JBaum » Thu Dec 14, 2017 9:32 am

I think your search for the source of the problem has been successful. Now I have to wonder if the Germans figure that needing a little bump at the end is considered within specs and the let it break in to working effortlessly like we're used to with the slightly worn and worn barrels we normally find to buy.

A few thousandths can make a difference, but if it locks fully with a slight bump, which the bolt actually has normally anyway, you're good.
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Re: MG3 Barrels, Bolts and Headspace

Postby Waffentag » Thu Dec 14, 2017 10:22 am

John, Great, then this solution is a major win for folks with short headspace. It beats sending it off to a gunsmith.

Good question on the German Specs, I found the below on the CMP Forum, The Germans used the FAL (G1) before they got the G3. So, it looks like max 308 length of 1.634 falls within the FN FAL Specs.
Not sure what is means as to the dimensions ammo that the Germans made. The ammo tested was "MEN"
I recall that I did test some LC 7.62 and it sat just above the top of case gage,

SAAMI .308 Winchester:
GO: 1.630 in.
NO-GO: 1.634 in.
FIELD: 1.638 in.

FN FAL:
GO: 1.6325 in. (FN & Brit/commonwealth. Canadian is 1.6315 in.)
NO-GO: 1.638 in.
FIELD: 1.640 in.

7.62 NATO (M14 US MILSPEC):
GO: 1.6355 in.
NO-GO: 1.638 in.
FIELD: 1.6445 in.


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