Building The Suomi M-31 Semi Auto
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The Prexis 80% receiver kit

The Receiver



It's all about the receiver!

Therefore, it's critical to be as precise and detailed as possible in building the receiver. Everything hooks to it and works off of it and you really only want to have to buy one of them!



First off, the receiver comes with a bonded on paper template. I've found that I cannot totally trust that the template was put on correctly! Very carefully measure across the receiver flat on the front. This the master referance point. Divide this flat in half crosswise. Mark the center. Ensure that the template is precisley lined up on this centerline.

If you're using a mill, you may want to use coolant liquid to preserve your cutting tools. I found that completely covering the entire receiver and template with clear packing tape sort of waterproofed the template. I also found that the water based machine oil coolant slowly seeped in under the tape so I had to do all of my machining in one setting. I also found that the machining pulled up the edges of the tape and obscurred the cutting line. To rectify this I used a razor knife to remove the tape and paper template over the places I wanted to mill. Just cut the tape and paper right on the line.

Even the drill and file guys should do this next step as insurance. Take a small hammer and sharp center punch and lightly mark every corner, contour and hole drawn on the template.

If in doubt, mark everything of interest. Just place the punch on the spot and give it a light tap. Mark it, don't create a crater! After everything is marked, if you want, you can completely remove the template. Then take a small steel ruler and connect the dots with a sharp scribe so you can clearly see the outline of each cut. Even if you don't plan on using a mill, the constant handling and frequent grinding on the receiver can cause the paper to come off, leaving you without a pattern. Marking all the key points is good insurance.

For the sake of simplicity, I started with the long slot on the bottom.

This is a straight forward 3/8 inch slot cut for the milling machine so I will quickly discuss the hand tool option.

Clamp the receiver in a good drill vice on a drill press. Use a bubble level to insure it's plumb in two axis. But, first check the drill press table with the level to see if it's level! If not, level it in two axes 90 degrees apart.. Use the flat spot on the front of the receiver as the place to check rotational level. Lay the level along the long axis to level it fore and aft.


Carefully drill, while staying inside the lines, each of the four corners. Use a 1/8 inch drill. This clearly sets the corners. Follow this by drilling a series of small holes with a step or center drll, right down the center line of this slot. Then take a drill that stays inside both side lines, 11/32 is good, and drill a string of holes that almost touch each other.

Go all the way down the center axis of the slot. Then use a file or Dremel tool to cut out in between all of the holes. Carefully hand file the sides to clean everything up. Stay inside the lines!

Use the same technique to do the top sight slot.

A side note here on the sight and its slot. The sight is used to guide the bolt in the receiver. It protrudes into the inside of the receiver and runs in a slot in the top of the bolt. Because of this it's VERY important that it's right on top of the receiver. A good idea is to index the receiver so that the flat on the bottom is sitting on a flat, square piece of metal. Then check to ensure that the top slot is really on top! The template could be off by a slight amount, but the sight must not be! See the Rear Sight section for more information.


Next would be the ejection slot. This one is a lot harder just because the steel is four times thicker. The same ideas apply here and milling is again straightforward. For the drill and file crew, use smaller drills, like 1/4 inch, drill slowly and use a bit of oil directly on the bit to cool it.








Warning! The ejector is DIRECTLY below this slot. DO NOT cut it off or damage it.!


Please note where the ejector is.




Putting a slight bevel or funnel shape on the outside of the ejection port helps prevent cases from bouncing back into the chamber and causing a jam.


While thinking about the ejector, it's important to remember later that the ejector will be too long and too tall. When you have the project to where you can lock in a magazine and cycle the bolt, you need to check to see if you can eject a loaded, chambered round. Do this without a firing pin in the bolt! I had to cut back my ejector length by almost a tenth of an inch and the height by .050". I use a Dremel type tool and a long skinny grinding stone - sold as a chain saw sharpening stone.


And now for the hardest of the receiver ports, the magazine well! This is by far one of the most critical of all of the steps. This opening must locate the top of the magazine so that the bolt can strip a round out and into the barrel while not hitting the feed lips. This area has a lot to do with jam free feeding while shooting! The front to back dimension of this opening must be such as to support the magazine with minimal play.

This port is shown on the template as two different areas. The inner portion is milled or drilled straight down into the receiver. Again - DO NOT HIT THE EJECTOR! Look into the inside of the receiver while in the final depth of drilling so you can see the tip of the drill or mill. The small square area in the front of this cutout is for the feed ramp portion of the front filler to fit into.




The outside shaded area (arrows, above) is then milled or filed/ground at a 45 degree angle without increasing the size of the innermost opening.



The photo shows using a machinist square to set the front flat at a 45 degree angle.

The width of the angled mag well opening is determined by the fit of a magazine in it. It will be about 1.150" wide at the outside. The leading edge of the larger opening will be about .720" from the large diameter edge of the step in the front of the receiver, where it steps down for the shroud. The formost edge of the small square area will be about 0.360" from this same step. See drawing above.

Don't allow the magazine to actually contact the bolt or damage to the magazine can result.







You will also need to radius the front corners of the finished magwell opening. These corners form part of the feed ramp system, so they need to be smooth and contoured. Here you can see sharp corners projecting into the path of a cartridge trying to feed into the chamber. Contour where shown on the left (arrows). Again, a Dremel with a chain saw sharpening stone is the tool of choice.

This step is best done after the front filler and feed ramp are fully fit. This allows a smooth blending of the receiver into the feed ramp.





Later, when you have the receiver fully machined, the rear sight installed and the bolt turned down, you need to test for easy bolt movement in the receiver. The two biggest problems are misalignment of the bolt due to positioning of the rear sight, which acts as the guide for the bolt, and extractor drag. Test this by putting the bolt, without striker but with extractor fitted, into the receiver, place your palm over the open rear end of the receiver and tip the receiver back and forth. The bolt should slide freely and fully back and forth inside.

To test to see if the rear sight and the ejector are not well aligned, remove the sight and retest. If this fixes the problem, revisit the chapter on the rear sight.

To test to see if the extractor is dragging, remove it and re test. If the extractor is the issue, use a large rat tailed (round) file to open up a channel in the small diameter portion of the forward receiver where the extractor is rubbing. Some Dykem (from Brownell's) on the extractor will help show where it's rubbing.

When in doubt - take less out! We will revisit this area later when fitting the front and rear fillers.





Just remember, please keep this safe and keep it legal!




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