Needed.   17mm drill bit.   Welder.   Lathe to make mandrel and turn outside after welding.

Precautions:  We don't want to create anything that might look like we are rewelding a receiver. 

Do not take off the fillers, as the front filler will be used as part of the "jig" to correctly position the front of the demill.   I used a chop saw to remove the excess receiver tubing at the rear. I made the mandrel from 3/4" hotrolled (A36)  I drilled it out 5/16" and then turned it on centers to a  little over 17mm to fit the rear of the trunnion tightly.  For the front, I turned a bushing that will sleeve it to 17mm.  This bushing will remain after we are finished and function to reduce the ID of the trunnion so a short SMG barrel will no longer fit.   Remove the ejector.  Locate the three pins (actually legs) and use a small punch.  Bump one a little then another then the third-rotating between them until you have removed it.  Sometimes, the front pin is not easily located because it is approximately where the cut is made.  Take your time because you want to save it if it is good because I know of no source for new ones.

 Clean all the sl*g and garbage off the demilled pieces, including the inside.  A round file will remove any metal that melted into the inside of the trunnion.  Don't worry about grinding some more away on the pieces because you need plenty of gap to do a good job welding.  In this case, unlike a sheet metal receiver, the wide torch kerf is a blessing but probably still not wide enough.  The front portion of the 17mm mandrel was slightly flattened on one side so it would slide across the top of the feedramp which  protrudes into the receiver slightly.  See picture below



Next, assemble the bushing, mandrel, and two pieces of trunnion.  Below, Bushing ready to drive into front piece.

Bushing pressed into front piece sleeves it to 17mm, the diameter of the remainder of the trunnion.  This gives us something solid to weld to and remains when we are through to prevent the use of the short smg barrel--allowing the retention of the takedown feature if we wish. ( You will have to turn a 16"+ barrel to fit the new /smaller trunnion diameter). 

Below, ready to slide together over the mandrel.

All content on Weaponeer is copyright protected.  The content and photos in this thread are Copyright 2008 to Weaponeer.net and Gundoctor.  Feel free to take for your own personal use, but not for reposting elsewhere or publication of any kind.

Edited by Weaponeer

Next, we need to align everything and get the spacing correct where the metal was torched away.  We do this by using the barrel shroud.  First, turn the front piece of the demill's trunnion into the shroud until is is in the correct position for when the barrel shroud is "locked on".   Place a couple of "witness marks" on both the front trunnion piece and the barrel shroud so you can insure it doesn't rotate as you get it asembled to weld.  The reason you haven't taken off the front filler yet is because it will help you get the gap perfect and the "rotation" of the 2 demilled pieces perfect.

simple scratched "witness marks" so I make sure everything stays aligned.  Notice the wide torch cut--which is good since it is hard to weld this deep/thick without a wide opening.

Edited by Weaponeer

Now we need to weld everything to the mandrel.  I am talking more than a tack but not fully welded.  I simply want to be able to secure everything together so I can remove the fillers--which I need to do to really weld the lower part of the torch cut and to be able to turn it after welding.  After tacking, remove the rear filler (I use a hacksaw--sawing it most of the way parallel to the receiver and then breaking it off with a pair of pliers).   Disassemble the front filler by removing the pins.  They are not "tight", they are peened into countersinks on each side.  You should drill a little bit in the middle of them from one side--then the peened portion will "collapse" inward and allow you to drive them out.

Below:  I have "tacked" the trunnion together, removed the rear filler, and am in the middle of removing the front filler.  After this picture, I ground the kerf even more--and don't be afraid to grind into the mandrel to get good edges on the trunnion pieces you can weld cleanly and easily. Notice how lucky I am on this one--all three holes for the ejector legs are visible and will be preserved--making reinstallation of the ejectior very easy.  If you are having trouble finding the ejector pins/legs, this picture should help.




Edited by gundoctor

Just for fun, I am welding this with only my 110volt Lincoln mig.  Of course, most of it on "D" (highest) setting, using 75% Argon / 25% CO2 at about 15psi.  There is a lot of building up to do near the shoulder, around the ejection port, and in the area beneath the front filler.  Make sure you have plenty of room to weld--and try your best to weld it solidly to the mandrel and bushing.   If you do this, when you drill out the mandrel it will be smooth and void free on the inside of the receiver--and you won't even be able to tell it is a reweld.   I repeat my earlier tip:   grind away enough of the trunnion pieces that you can see the mandrel easily and can weld them too it.   My gap between the two pieces was about 3/8-1/2 inch wide on the outside dimension sloping to 3/16-1/4" wide at the mandrel..   When you are welding--weld some here and some there to keep from warping it.

Below everything is welded and built up.  It is solidly welded to both the mandrel and the bushing (and bushing to mandrel), effectively making this a solid receiver and not contraband.   At this point we can turn down the excess weld.

It will be easy to remove the mandrel by drilling from the front with the 17mm drill bit.  However, do not drill out the mandrel prior to installing some sort of blocking rail or other change to make it semi only compliant and prevent someone from thinking this is a smg receiver.




Edited by Weaponeer

Put it in the lathe and turn to original outside dimensions and clean up the shoulder on the front the bears against the barrel shroud.   I also turned some off the back.  This should be turned to fit the tubing you are using for the receiver.  If using a blocking rail, you might use  original ID tubing.  If using smaller tubing, simply turn the rear of the trunnion to the proper diameter to fit.   The receiver tube will be welded on and then turned smooth.

Semi Method 1 (Reduced ID receiver):  At this point, you would grind away the tack welds attaching the mandrel to the rear of the trunnion, and turn the rear to fit the ID (reduced from original) of the tubing you are using.  Slip the tubing over the turned down portion and weld.  AT THS POINT, you can remove the mandrel by drilling from the front about 1 3/4" with the 17mm drill and you will have a semi-compliant receiver since you used the reduced ID tubing and a FA bolt will not longer fit. 

Semi Method 2 (Blocking rail with original ID receiver):   At this point, you must weld the blocking rail to the rear of the trunnion.  It should be a a design such that an original FA bolt will not install and work.  My design welds to the trunnion and then to the receiver tube.  You can put the blocking rail wherever you want to--but remember where it is because you will have to slot a semi bolt for it later.

AGAIN:  Do not drill out the welded in mandrel until you have made your semi only provisions.    

DON'T FORGET:  
    1.  Do not weld the blocking rail, if you are using one, to the mandrel, only to the trunnion and receiver tube.  If you weld it to the mandrel you won't be able to remove the mandrel by drilling only the front 2" or so.
    2. Do not forget to grind away the rear tack welds (mandrel to trunnion) before welding on the receiver tube.  If you forget this, you will have to drill the full length as well--and that will take a long bit.
    3.  Do not slot your bolt for the blocking rail prior to making all the other semi-only provisions (to the bolt).  If you slot the bolt for the blocking rail prior doing the semi machine work you might be accused of making an illegal NFA firearm.

The ejection port can be cleaned up using an end mill or a grinder / dremil after the welded-in mandrel is removed.  



Edited by Weaponeer

Looks  Good , like to see more . Thank You, Jay

Use a grinder or lathe to remove the tack welds at the rear.  Assuming your semi provisions are in place, you remove the mandrel and reactivate the trunnion by drilling it out from the front with the 17mm drill bit.  Since most of the welding is within about 2.0 inches of the front of the trunnion, you won't have to drill too deep to release the welds.  As a matter of fact, once you have drilled though the welds the drill bit will simply push the mandrel out the back of the trunnion.  

After removing the mandrel, I used a 3/8 endmill to clean up and reshape the front of the ejection port.  Use the existing ejection port and  set the upper edge of the opening vertical in the vise, then set the mill where it is just touching it.  Mill towards the front and then 1/2 way towards to lower edge of the port.  Then rotate the trunnion in the vise to make the lower edge vertical and repeat.  The ejection port will then be "perfect".  You could do this with a Dremil, but it would take more than 10 minutes and not look as good.




Edited by gundoctor

I am not trying to keep Mike at Prexis from selling the aftermarket 80% receivers, but when doing something like the SuomiThompson build I hate to spend that much money and then have to heavily modify it.  Based on how easy it is to reweld the trunnion portion using this method, I would call the demilled receiver pieces a "60%"--It takes about 3-4 hours to put the front two pieces together properly.  Well, it might take you 5-6 the first time, and about 2 hours the 3rd time you do it.

The neatest thing about the mandrel / bushing / front filler / shroud method is that is becomes a "jig" that allows everything to be aligned and spaced correctly to weld.  Then, the front filler can be removed to allow lathe finishing of the trunnion and receiver.  Then, the front filler can be reinstalled by using the barrel shroud and the feed ramp notch (preserved in most demills and your reweld) as a "jig" to reposition it for welding.


Edited by gundoctor

Here is the remainder of the mandrel after it is drilled out from the front.  As you can see, only about 1 3/4 inch is drilled before it simply pushes out.  Also note where the tack welds were ground away at the rear prior to installing the blocking rail and/or receiver tube.   When you drill it out from the front, it will simply push out and fall out the rear of the receiver.

Note:  The unturned portion at the rear of the mandrel served no purpose.  In the lathe, I clamped directly onto the trunnion, not the mandrel, with my 3 jaw chuck.  Or, when I reversed it, I used a live center in the centerdrilled bore of the mandrel.  Hope this makes sense.



Edited by Weaponeer

Very good job!


Having done a complete re-weld of a Beretta 38A receiver, I find this approach most interesting.  Really good idea! 

If there is enough interest, I might have some "kits" made up to reweld a trunnion for a dummy gun or semi build as shown in this thread.  The kit would include the following:

17mm drill bit:  Turned down enough to allow you to use in a 1/2 drill chuck
Mandrel:   long enough to use for 2-3 builds
Bushings:  2

You can do this rebuild using this kit without a lathe by grinding off the welds:  of course, it will not be as pretty unless you are really good with a grinder!.  As an option, we could include reduced ID rear receiver tubes (to fit Tommerr's bolts) already threaded for the endcap. 

Instead of posting your interest here and lengthening the thread, send me a PM with your thoughts on my making reweld kits available.  This would be thru a "Group Buy" and will be more of a "friends helping friends" service, not a profit center for me or weaponeer.net

Once we've got a completed semi trunnion or upper receiver in our hands, there is a lot we can do with it in addition to building a semi M-31 clone or a dummy gun, and those threads will be "coming soon"


Edited by gundoctor

Didn't work on this much the past few days, but I did get the front filler cleaned up to reweld.   I determined that the original pins were a little less than 3/16", so I simply reamed the holes to 3/16" and used some new pins. I decided that two thicknesses of duct tape where the right spacing for the filler to the barrel shroud, and the rear of the filler is properly spaced by the notch in the trunnion and its radius.   I don't like the way the front filler welded (no pix).  It seems like some funky type of steel or maybe the torching episode messed it up.   I am going to take it off and preheat everything before trying to reweld it tommorrow.




Edited by gundoctor

Yes , interested -like to see more . "friends helping friends"  is a Good Thing . Thank You, Jay

Gundoctor, Have a question for you. Maybe I'm missing something here...Why do you weld the mandrel to the reciever then mill it out and drill. Why just not weld it? Or does it hold things in place while welding other parts of the trunion? I would like to do one, Also do you think a Lincoln 125 welder would handle the trunion? That's what I use. Please let us know why you do the welding od the mandrel to the reciever welding. Thanks...George

I weld the mandrel to the trunnion for three reasons.  The main idea is that it is the "jig" than aligns and holds everything while you weld.   There is a lot of welding to do--and big openings at the mag and ejection port--the mandrel also helps keep the trunnion from warping as it cools.

1.  It allows you to align and index everything and then not have it shift around.  It would be nearly impossible to weld this straight without some type of fixture to hold it and keep it from warping.  In this case, the "jig" is internal and sacrificed.
2.  It allows you to remanufacture the demill trunnion without transitioning through something that looks like a partially rewelded smg receiver.  You can do your semi mod's before removing the mandrel.
3.  By welding to the mandrel, when you drill the mandrell out the inside of the receiver will be smooth and solid.  If you weld without a mandrel of some type then it would be difficult to clean up the inside of the receiver  (I have done this--you can't use a drill--must grind--because the unevenness of the welds protruding into the trunnion will cause the drill to go off center.  With the mandrel, it only takes less than a minute to drill it out--and the inside of the receiver will be "finished" when you do.   You could do this with a copper mandrel and reuse it, but I don't think that represents any savings in time as the mandrel can be made in a few minutes.

I think your welder will do it.   I am considering preheating my next one to see if it welds even better.

I'm saving this one to the personal archives! I'm still fairly new to welding, but this is a great idea, and I might just have a use for the front section of my suomi chunks now

Thanks Doc , I thought i would have a hard time locating something on the suomi trunnion but this is exactly what i was looking for , and its from 2008 . unbelievable .. Thank You !!!!!

Gundoctor is a true craftsman. Please do a simple stress relief after welding. Welding goes from liquid steel to solid in a heart beat. Receivers are a medium alloy steel. You can produce major stress factors. Just pop it in your kitchen oven at 350F for one hour. That will soften nothing and make your build much safer.