MP-44 trunnions update

We finished the first 6 test sample MP-44 trunnions to the buyer. This is an update report on the product. At the time we made these we had no flats to test them against so certain measurements could not be verified against an actual matching part. This lead to a couple of minor dimensions that would have big effects.

The first in the button hole that is one of the points that a sheet metal pressing sets into the trunnion properly. The hole dimension was correct, however its position was off by a very slight amount.

DSC_6306cwsA .5mm in one direction and a 1mm direction in another. Small but important. Now with the solid models changed this will no be a problem in the production run.

The second problem was one of our radius was off. This radius allowed the flat to set next to the trunnion.

DSC_6305cwsAs you can see in the picture the radius is incorrect and not allowing the flat to set properly to the trunnion. Most likely this was a file and fit part of the original manufacturing process.

This is what it should look like.


The remaining test pieces will be corrected and sent to Pete to test prior to making the final production run.

Some of the differences that we have come across that need to be addressed when reverse engineer a part. What are the tolerances that we have to deal with. An example of this is the trunnions that we are working on. We just receiver a second cut original cut trunnion and have found the following differences. First with the original trunnion that we received there is .010 difference in parallelism between the two sides. the width difference between the first trunnion and the second one is .015 measured over the complete part in both cases. Now between our part and the first one that we started with we found our parallelism to be .003 and the difference between the two parts .003. So now the question is what is the max width. Are we over because the first part we tested was at max or are we in spec. The next step is to put the part in a receiver and check it out.

Reverse engineering is a great deal of work. Here at Gun Lab we do a large amount of it ranging from simple to almost impossible projects. In fact we currently have one our plate that has taken 2 years to get to basic drawings, now comes the hard part of checking it against every other component. Just to grasp the project not every minute has been spent on it and lucky the owner of that project is not driving me crazy and understand the complexity of the task he is asking. However it will be perfect when it is done. Everything has to checked, re-checked and then checked again, not an easy task and sometimes very frustrating but in the long run worth it.


3 comments to MP-44 trunnions update

  • Pete F.

    More on the work on the receiver block dimensions
    Measuring and checking several rear sections from cut receivers shows a fair amount of variation in the placement of the “button hole” That’s the divot drilled into each side of the rear of the blocks. They vary enough as do the barrel pin hole and the ejector pin holes that they will need to be located and drilled by the builder using the new block.
    I can outline a method to “find the right place for that divot. Or it can be done using an existing block remnant that would have been removed from the cut rec.

  • Chuck

    Thanks for the info Pete. We also checked the trunnions here and found a large difference between them as well. In the 1940’s they did not have G.D.and T. Infact it was due to WW2 that this system of stacking measurements came about.

  • IAC

    Sir, If I may ask a question regarding the M44 rear stock? Otherwise please direct me to who might know.
    On the STG/MP 44, was the rear stock merely butted up against the rear receiver cap and screwed from within the cap; or is it inserted INTO the cap ?
    In photos and schematics I seem to see both.
    Thanks, IAC.

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