MG 42 Build:Headspacing a New barrel?

[MarkoStefano]

Greetings, I am new here and so this is my second question of many I am sure! I have an Apex M53 kit which comes with the barrel extension complete with a smoldering barrel stub! But that's another problem! So I purchased an new US made 8mm barrel from Apex this past week and it arrived yesterday! It is beautiful! Now I haven't gone far at all in the project, just have everything cleaned up and I have refinished the buttstock. But I gather that I will need to use the barrel during assembly to help establish the proper relationships of the shroud reconstruction and the rear BRP receiver. So I have read here that, and please correct me if I am misinterpreting things, that the headspacing procedure (out of the gun) consists of inserting the bolt (without firing pin) into the extension, with a new live cartidge, ensuring the rollers lock outwards. Then screw the barrel in place until it just makes contact. I assume that this all can be done alternately using headspace GO and NO-GO gages? I would need to buy those, would I also have to have a field gage? Does the barrel usually screw in all the way to the shoulder, or does it make contact somewhere before, or after, requiring chamber reamer? These Apex barrels are chrome-lined, so that is not an option! Well that's a lot of questions in one post, and please forgive my rambling! Any help is greatly appreciated!

[DARIVS ARCHITECTVS]

The barrel screws in all the way to the shoulder. Now you will need a Go and a NoGo gauge. The field gauge will show you when the head spacing is beyond the NoGo stage into the DANGEROUS HOLY SH1T DON'T YOU FRIGGIN' PULL THAT TRIGGER stage. The headspacing is way to large, and casings may rupture and you will have a real f*cked up day.

1) If you are re-head spacing a barrel, you will want the head spacing set such that the bolt roller fully lock with the Go gauge in it, but stop JUST SHORT of moving FULLY outboard (short of fully locking) with the NoGo gauge in the bore.

2) Start with an assembled barrel and locking piece fully screwed onto it. Check the head space. If it is too tight (Go gauge will not allow full bolt lockup) then use your reamer to increase the head space in micro small steps until it is. Don't ream too much such that the NoGo gauge can allow full bolt roller lockup.

3) If the head on your assembled barrel is too large, then you have to take the barrel off, put it in a lathe, and turn a bit off the collar's seating surface, reassemble it with the locking piece, and re-measure the head space clearance. If still too large, repeat this entire step once again until you are at or below the desired head space measurement. If you turn off a significant amount off the collar and go too far, then the bore was so sloppy that the barrel may be useless, but that would be rare. If your head spacing is a bit too small, go to step 2.

4) Once head spacing is set properly, making sure your polish the chamber with the proper tools so that extraction is glass smooth even with a dirty bore. It makes your gun reliable.

MG42.us Forum guyz: If I missed anything important, please chime in.

[Blanksguy]

....remember to torque down the barrel into the barrel-extension and "stake" it at the notch in the Barrel-Extension forward face. If the barrel-extension does not have a "notch", then grind a small notch to stake the barrel to.

I don't remember if it was early MG42 barrel-assemblies were not staked, or just during the first-time use of the barrel-extension.......and after, they would be "notched" for staking of barrels.

Regards, RichardS.
US Army, Retired

[spareparts]

do the new barrels from Apex generally require any headspacing work in terms of reaming or facing?

[Blanksguy]

.....sometimes...........it will depend on the barrel-extension.

Regards, RichardS
US Army, Retired

[along]

So I have a barrel extension and new 8mm barrel from apex.

I understand the barrel screws into the extension, is torqued down until headspace gauge fits and no-gauge does not I believe.

How exactly is the barrel torqued into the extension, with a tape wrench and vise? I've never worked with screw in barrels so its a first time for me.

Additionally once properly headspaced, where exactly does the stake go without damaging the inner threads?

Thank you for any help.

[JBaum]

You screw the barrel extension and barrel together, then check to see what headspace you have. The barrel has to be tight in the extension regardless of the headspace. Then you perform the steps above to get the correct headspace if it's not within tolerances.

Reread the messsage above from DARIVS ARCHITECTVS.

If you have more than one bolt, check headspace with all of them. A barrel may be ok with one bolt, and not ok with another. It's a lot less trouble to try a few different bolts and find one that fits, than it is to modify the barrel to fit a bolt.

And it's a LOT less trouble to pay a couple bucks extra and buy an original barrel, which 95% of the time is within tolerances.

The seam where the barrel and barrel extension go together is staked with a centerpunch to keep them from unscrewing from each other.

[along]

Do you screw the barrel tight enough in by hand or are any tools specifically meant for it?

are there particular gauges I should buy for headspacing the mg42, as in brand?

I have one bolt and bolt head. I am doing a semi-build , so I don't think I'll need more than one barrel.
I bought one of the semi-auto bolts from apex, and was going to take the bolt head off my original bolt and combine it with that. I still have to to drill the firing pin head to take the semi-auto firing pin.

Ah for the staking I thought a pin or something actually was punched in. What size centerpunch if there is one?

thank you for your help

[JBaum]

A band type pipe wrench is good for screwing in the barrel. You could use a regular pipe wrench, but the barrel gets chewed up. The barrel is not pinned, just screwed tight and staked.

No specific size on the center punch. They have a tapered shaft, but the point tapers from about 3/8' across down to the point.

Headspace gauge brand doesn't make a difference.

[along]

my barrel extension has a staked mark I assume from its previous barrel, should I use the same stake mark or make a new one. If I make a new one where do I make it?

is there a specific mg42 go and no go gauge? I was reading the headspace for mg42's is differnet than say a kar98k.

thanks

[JBaum]

Headspace is the same for a 98k, and an MG42, and an MG34, etc., the technique for using the gauge is just different.

Stake it where ever you like. The original place is fine. Just flatten a little of the barrel flange into the original depression on the barrel extension. The idea is to keep it from unscrewing itself. That's all. No magic here.

[along]

Ah ok so it seems 8mm headspace is 8mm headspace.
Excuse me for my ignorance but is the barrel flange on the barrel extension or barrel itself next to its collar?

thank you for your help

[JBaum]

The barrel flange is the collar.

[along]

ok makes sense,thanks for the help

[along]

ok so I just bought a tape wrench to screw the barrel into the barrel extension. Is it ok to just put the extension into my vice? I do not want to damage or bend my extension. I didn't know if there is a jig or something I should use like 2 2x4's to not tighten the barrel extension against the vice.

thanks for the help

[JBaum]

Padding with a couple boards would be good. Clamp mostly on the solid part of the extension, leave the wings sticking out a bit for protection against bending. Don't stake until the headspace is good.

[along]

ok sounds good, gonna go do that now. also is it necessary to buy a field gauge along with the go and no go gauge?

[along]

so I'm almost done dremeling out enough material from the front shroud for the front receiver bushing to pop in, does the bushing go in with the milled out space inside the bushing point to the right of the front sight post?

thanks

[along]

so I tried out my go and no gauges for 8mm, I bought them from Forster. I have my original barrel extension with cut stub and a fresh one I bought and have installed on my barrel. I first tried the original extension, the go gauge closes with when rollers extended are 4mm away from the roller slot end. I also tried the no go gauge and the bolt closed on it. I then tried my now installed (I have not staked it) barrel extension on new barrel with my bolt. It closed on the go gauge with the rollers 3.5mm away from the end of the slot. I also tried the no go gauge and it closed on that as well. is .5mm of difference enough to cause problems? Additionally is the no go gauge closing on my original barrel extension bad or is my no go gauge to SAAMI spec not meant to gauge it?

I also tried my go and no go gauge on my mauser, it closed on both.

any help is greatly helped, will my gauges work?

[JBaum]

"the go gauge closes with when rollers extended are 4mm away from the roller slot end".

Uh, no. If the rollers are 4mm away from the ends of the roller slots, it didn't "close".

Headspace is measured when the rollers are FULLY OUTWARD, at the ends of their slots. If the rollers aren't fully outward, the bolt isn't closed, it's ALMOST closed. Measuring how far the rollers are away from the ends of the slots just gives you an abstract idea of how much too tight the headspace is.

If the rollers won't go fully outward on the go gauge, headspace is too tight. The bolt isn't considered "closed" until the rollers are fully outward at the ends of the slots. The equivalent for a rifle would be not having the bolt handle able to turn completely down with a gauge in the chamber. The rifle isn't ready to fire unless the bolt handle is fully down. Same for the MG42, only it has rollers in slots, instead of lugs in the receiver grooves. The principles and methods are the same, really.

You wouldn't fire a rifle with the bolt fully locked (handle fully down), and you don't want to have the MG42 fire with the bolt not fully locked (rollers FULLY outward). It is embarrassing to have someone picking brass shrapnel out of your arm.

I suggest you read (or re-read) the above messages, and the link below:

viewtopic.php?f=39&t=4907&p=91369&hilit ... ace#p91369