Booster cone hole sizes

[Intruder196]

Can anybody tell me the correct size of the hole for a 8mm forcing cone? I am trying to figure out if mine is for 8mm or 7.62. I really only plan on shooting 8mm.

[ak47dennis]

Intruder196, for 8mm you can use 10.5mm or 11.5mm I have a few different sizes forget what they all are but I know they go up to 14 mm. A majority of these different sizes are post war.

I believe that all the 9.5mm are post war for 7.62, DONT use this in you semi with 8mm.

[JBaum]

8mm uses a 14mm nozzle, or to increase the rate of fire, an 11.5mm nozzle can be used. The 11mm nozzle is used for .308. The original German manual this information comes from doesn't list nozzles with holes smaller than 11mm.

This information is in the German MG42 manual H.Dv. 216/6.

[Blanksguy]

The Germans refer to these "booster-cups" as a "nozzle". Basically you have a cone-shapped "cup" under the screwed on Flash-Hider....these came with different size exit hole to the front of the booster-cup. Depending on what caliber/pressure/diamerter bullet the gun was set up for...it would require a different size/diameter exit-hole in the booster-cup.

Who has a good sheet/list of what sizes were made?...and by what countries? (IE: WWII German vs. Yugo. M53 manufactured like a German WWII MG42 vs. MG1 booster-cup exit-hole sizes) ?

[ak47dennis]

The Germans refer to these "booster-cups" as a "nozzle". Basically you have a cone-shapped "cup" under the screwed on Flash-Hider....these came with different size exit hole to the front of the booster-cup. Depending on what caliber/pressure/diamerter bullet the gun was set up for...it would require a different size/diameter exit-hole in the booster-cup.

Who has a good sheet/list of what sizes were made?...and by what countries? (IE: WWII German vs. Yugo. M53 manufactured like a German WWII MG42 vs. MG1 booster-cup exit-hole sizes) ?

Blanksguy

I have the following in front of me:
-9.5 mm
-10.5 mm
-11.5 mm
-14 mm

Not sure on any of their origins, but I do have one of the 11.5mm nozzles that has a serial number on it that matches a top cover I have.

[Reichpapers]

Though we have many sizes and we are not very sure what they are used for,one thing is certain...each size had a specific use. Myrvangs book states that the early MG42's used a 10mm cone, then later standardized into the 11.5mm cone. He also states that the MG34 used an 11mm cone, but also had a 9.5mm cone for use with their aluminun core training rounds. With this we have possibly accounted for 3 of the 4 sizes....In addition, the .308 round probably takes the 9.5mm. But the 10mm and 14mm??? Maybe it goes with the Yugo copy of the MG42. Keep in mind that book is by no means a final answer, but it is the only book with cone size references available. Concerning the 11.5mm cones on the 34. Either the book is incorrect, or there was a postwar need (whichever country these are being imported from) to increase the size to 11.5mm.

[Blanksguy]

Myrvangs book on the MG34 and MG42 was a good start in sorting all of this out....I hope that he will expand on those subjects and others.

The MG34 and MG42 use different Booster-Cups.......different in that they have different outside diameters and shape. The different bullet exit-hole of these Booster-Cups are made at different diameters for two basic reasons:
1: Diameter and bullet-weight/ammunition-loading of bullet being fired.
2: The operating-"back-Pressure" requirements to operate the system (IE: Make the barrel move reward with bolt attached, under recoil-spring-pressure).

What is bad is that the faster/harsher you want your gun to operate....the smaller-restriction for the Booster-Cup (within limits of the bullet-diameter). Such as would be the case of an MG42 firing at 1,000 RPM with a 11.5mm restricted-Booster-Cup......will fire faster with the 10mm or 10.5mm Booster-Cup due to more pressure being directed back onto the muzzle of the barrel during the short "flight-time" the bullet takes to pass through the Booster-Cup.

This is compounded by the problem of the size and shape of the pressure-relief openings at the rear of the Booster-Cup of the MG42. The width and height of these six pressure-relief-slots will determine how much "pressure" is dumped as the barrel-bearing is forced to the rear with the barrel's muzzle attached.

Now to really confuse you.........measure the length of your barrel-bearing....and how far it protrudes from the front of your MG42 with the Flash-Hider and Booster-Cup removed. Let us know what you measure...as this determines "when" the pressure is dumped when the front of the barrel-bearing passes the openings in the Booster-Cup during recoil/operation. This is the "timed" period of when the pressure is released at the rear area of the Booster-Cup. At this time, I have:
1.970" length Barrel-Bearing.
1.865" length Barrel-Bearing.

This is where a good list of "what-fits-what" for caliber-conversions would really come in handy. "German Machineguns" by Musgrave (?) talked very briefly about the differnt sizes and lengths......but really did not go into much detail. This may be the case for some guns firing faster or slower than what was desired (1,200 RPM ?).

What sizes have others measured on their MG42s (and MG34s) and in their "Parts-Kits" for:
1: Booster.
2: Barrel-Bearing.
.....and if possible, let us know if these were WWII Greman-"proofed", or Post-War...and maybe by what country?

TIA: RichardS in Michigan.

[ak47dennis]

Blamksguy, that’s a terrific overview.

I was not aware of the different barrel bearings. I checked the distance sticking thru the tip of the receiver and it is .640 or 16.25 mm. I would have removed it but they do not come out with the front sight & retaining finger installed.

[Blanksguy]

I am still new to this board....but use to post some useful information on the old MG42 Board.......If you would like, I can gather up this information to post for you.......or gather it up as a reference and send it to you for posting. Being old "Army", not very computor literate.

I have done a lot of research/test-firing with Blanks during the course of my "Blanks-Sales-Hobby"....and there is quite a bit of difference in opertional issues (restrictions with the match-up of different parts)...and in the RPM (Round/Blanks per minute).

I was contacted by a couple of young men about BFA Blank-Firing-Attachments/Blanks-Adaptors).
I have a lot of information and drawings for the MG42 and MG34 using both the two-pcs. Blank-Barrels....and the Blank-Firing-Booster-Cups for both guns when using a standard barrel. ...along with some feed-tray modifications to get the MG34 running well with 7.62x51mm/.308 Win. Blanks.....even those cheep British/Austrian Blanks can be made to work well (although not as loud. As I do not have a "scanner", I would need to send this to someone higher up on the "food-chain" (web-master, or ?) to get it posted somewhere ?...I would have to mail it to you.....have you check, it then post it where others can access it to make their own Booster-Cups and 2-pcs. Blank-Barrels.

Presently we are working on ways to modify the Blank-Barrels and/or Booster-Cups to increase this noise level.
I can be contacted "off-line" at:

Blanksguy2001@chartermi.net

Let me know what you would like to do, RichardS.

[JBaum]

Blamksguy, that’s a terrific overview.

I was not aware of the different barrel bearings. I would have removed it but they do not come out with the front sight & retaining finger installed.

My barrel bushing is removeable.... just lift the flash hider detent lever out of the notch, and the bushing will release and come out the back. It needs to be removable for cleaning.

[Blanksguy]

Yes, MG42 Barrel-Bearings are removeable by removing the barrel....then lifting the Flash-Hider-Catch (just forward of the front-sight).....and sliding the barrel-bearing out of the receiver from/towards the rear of the gun.

To re-install the barrel-bearing is nothing more that doing the same things in reverse order.....remember to raise the Flash-Hider-Catch to allow the barrel-bearing to move fully forward.

Also....Remember during instalation that this Barrel-Bearing has a large slot cut into it that must face the right side of the gun for instalation of the barrel. ....and that the barrel-bearing has four "keys" that must match the same "key-ways" cut into the receiver.

On a side note......"Important"...it you are making up a 2-pcs Blank-Barrel for firing "Blanks".....it should have a collar at the rear to keep forward portion of the 2-pcs. Blank-Barrel from moving forward and shearing off these four keys on the barrel-bearing....OR messing up the key-ways in your receiver.

Be safe, RichardS.

[ak47dennis]

Yes, MG42 Barrel-Bearings are removeable by removing the barrel....then lifting the Flash-Hider-Catch (just forward of the front-sight).....and sliding the barrel-bearing out of the receiver from/towards the rear of the gun.

To re-install the barrel-bearing is nothing more that doing the same things in reverse order.....remember to raise the Flash-Hider-Catch to allow the barrel-bearing to move fully forward.

Also....Remember during instalation that this Barrel-Bearing has a large slot cut into it that must face the right side of the gun for instalation of the barrel. ....and that the barrel-bearing has four "keys" that must match the same "key-ways" cut into the receiver.

On a side note......"Important"...it you are making up a 2-pcs Blank-Barrel for firing "Blanks".....it should have a collar at the rear to keep forward portion of the 2-pcs. Blank-Barrel from moving forward and shearing off these four keys on the barrel-bearing....OR messing up the key-ways in your receiver.

Be safe, RichardS.

My bad , I forgot about lifting the flash hider retaining finger. I need to clean my 42 today was at the range yesterday and went thru 1/2 case of Romanian 8mm, BOY my 42 loves this stuff only one FTF. Anyway while it’s apart for cleaning will measure my barrel bearing.

[M/W]

My barrel bearing measured 9.3 mm, the cone came out at 10.16 mm

[Abwehr]

I read this thread a couple of days ago, but could not measure the Booster until I got home. I cheked my 8mm MG42 and the Booster is 11.5mm I.D.

I also measured my 7.62 NATO MG1/2/3 Booster and it is 10.1mm I.D.

[Blanksguy]

Information needed is:

1: Diameter of the hole in the front of the "Booster-Cup" that sits under the Flash-Hider.......and if it has any markings (IE: WWII German Markings, engraved or stamped number/letters).

2: The length of the "Barrel-Bearing" .......not only the length of the barrel-bearing when removed from the receiver.......but also the distance it protrudes forward of the receiver when a barrel is installed.....and if it has any markings.

We will tabulate our findings and post what we believe is a basic-chart for the different calibers (IE: 8x57mm vs. .308) , early vs. late WWII German, and "standard" WWII/Post-War 8x57mm parts. This is all based on the differnt readings/measurements from Board-Members...and that written data from books like "German-Machineguns" by Musgrave......the newer Collector-Grade-Book: "The MG34 and MG42"...and any manuals that we can reference.

Notes:
A: It would help if you could let us know if these parts are from an 8x57mm Parts-Kit or a .308/7.62x51mmNATO Parts-Kit.
In reference to these parts...it will be helpful to know if they look "new", or "well-used", or "refinished". This will let us know if we need to average in a "used" factor for larger diameter holes (IE: A 10.2mm hole would probable turn out to be originally a 10mm sized hole).

B: When measuring....try to use a precision-dial-caliper....and check to insure it is zeroed prior to measuring.

C: To remove the "barrel-bearing"......first remove the barrel-assembly.....then lift-up the flash-hider catch at the top-front of the receiver and remove barrel-bearing to the rear from inside the receiver. (Also note that you may have to raise the front-sight-assembly on some models so that the barrel-bearing will clear).


Be safe, RichardS.
Blanksguy2001@chartermi.net

[Reichpapers]

May I also suggest measuring the hole from both axis's...just in case it is keyholed.

[medved]

My 42 has an 11.5 hole diamter and will reliably run 8mm and 308

[Gulfstream4]

I was reviewing some stickies and was wondering if anyone would want to follow up on this. It would be nice to tabulate findings on the use of different booster sizes, barrel bearings with different ammo etc...

Any thoughts?

[42rocker]

I was reviewing it also the other night and noting the fact that before us the last posts were about 6 years ago I think the project is dead or sleeping big time. For myself I just reviewed what I have and I'm going to the range with 11.5mm and a 13mm cones. See if the 13mm gives enough back pressure to function everything. If not I have the 11.5mm as back up. Also I will be running semi auto. I think that was one of the things that was left out of the above. How it runs semi or Full auto.

My cones went from 10.3 to 11 to 11.5 to 13... The "modern" yugo m53 ones were all 13mm. I looked at about 40 cones and those were my 4 sizing groups.

Later 42rocker

[TactAdv]

My cones went from 10.3 to 11 to 11.5 to 13... The "modern" yugo m53 ones were all 13mm. I looked at about 40 cones and those were my 4 sizing groups.

Later 42rocker

From period documents, the nominal size for the original MG39/41, carried over to all the manuals issued for the adopted MG-42 was 11.5mm.

There is a continuing later-day mystery why the empirical physical samples existing show such a wide variety of sizing, as 42Rocker indicates above and which I totally agree with. In researching a lot of the industrial history of this weapons system program, it became evident, and this is backed up by the existing historical records from many sources, that the Germans recognized these guns, for a multitude of technical reasons, exhibited significant individual personalities and did indeed have widely varying cyclic rates. it has thus been the target of significant speculations that the standard, and simple, way to 'cure' these individual cyclic rate variations was by installing different booster nozzles in such guns that showed "unacceptable" deviations from the desired combat specification of "15 shots per second", which was ALWAYS the desired functional standard. Up or down, the rate could be varied fairly easily and consistently in individual guns by merely swapping in a nozzle that was more less constricted.

This was almost certainly the result of largely undisclosed differences in the mating of individual bolt head and barrel extension combinations, which the wartime manuals and documentations explicitly discussed but did not further pursue, basically, if a certain combination of bolt head + barrel extension, when mated, produced "more" or "less" contact interference it would then take "more" or "less" recoil force to unlock the mated parts appropriately. This was apparently recognized, but I have never seen a reasonable explanation put forth as to WHY this situation existed as these parts were all made to interchangeable tolerances.

In any event, such a condition can be readily seen to impact the basic cyclic rate of the gun, individually speaking. And as such, the need for, or historical existence of these varying booster nozzle apertures makes sense, even though the documentations indicate both a "standard" size of 11.5mm and a normal cyclic rate that should be present with that size installed.

There is also now known, some indications that the smaller sizes of booster nozzles found in certain MG-34's and MG-42's may have been purposely installed in guns destined for, or used in, areas where combat at extreme low temperatures were found or expected, the increased back pressure being desired when these guns were fired completely dry, devoid of any lubrication which was the combat norm for such conditions, and more importantly as was discovered in a report of MG-34 problems, that the extra impulse from the tighter nozzle aperture could be counted on to function the guns mechanism if it were already frozen up, prior to warming up with use. The notion then, was that it was better to have a gun that actually could be counted to fire when needed, and worry about excess battering later. Obviously, much has been lost to time in this discussion unfortunately.

-TomH