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Thread: Suomi build

  1. #1
    gunco irregular moleman's Avatar
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    Default Suomi build

    Since I chronicled my first suomi-ish build that started out as a sgn-9 build I figured I'd start a new thread since build #2 is definitely going to be a suomi when I'm finished. I started with a suomi kit froms copes and 1.5"x.188" wall thickness 4130 steel tubing from wicks aircraft supply. I turned the approx 10-3/4" receiver tube down to 1.465" between centers and threaded the last .450" (the original suomi's have .4" threading) of one end 20 tpi for the end cap. I noticed that the front 2-3/8" section measured 1.455" diameter so I turned it this receiver down to that measurement. On my next one I will leave the front section full size until I weld in the barrel bushing. That way I can just turn down the welds and hide them. Next I took a cylinder hone and lightly cleaned out the receiver tube. I then took a 5" section of 1-1/4"x.250 wall tubing and cut it into a barrel bushing. The section that fits in the receiver tube needs to be 1-2 thousands over the i.d. of the receiver tube for a press fit. I copied the lug area from several different suomi receiver sections as well as using measurements from my first build for overall length. The length from where the barrel collar rests (not including the indexing nub) in the front to the back where the bolt is stopped on my first build was around 4.24" to obtain the proper headspace. I left this barrel bushing at 4.270" (with the extra length in the section that goes inside the receiver tube) so I can turn it down later to get the correct headspace. Since this is a blowback type gun it isn't as critical as a locking action, but on my first build if I let the leading edge of the bolt contact the barrel it would buckle the cases of my dummy rounds because of the short headspace. I used a max trim length 9mm dummy round as a guide and added a few thousands for good measure. This left me with 4.245" for my barrel bushing length.
    The inside of the barrel bushing is around 3/4" and a suomi barrel is around .78". To keep from getting a sbr violation on my first build I: lengthened the barrel to 16-1/4"; turned the barrel shank down to 3/4"; enlarged the indexing nub to 1/4" and welded on the barrel jacket since I didn't cut the lugs into it. Even without welding on the barrel jacket there was no way an unmodified suomi barrel was going to fit in the receiver. On this build I am using the same larger indexing nub, reduced barrel shank and welded on extension to avoid a sbr violaton.
    Since this is a work in progress I'll add some more pictures over the next few days to bring the build up to where I'm at. Here's a pic of my first build That I still need to put a finish on and some pics of the receiver and barrel jacket of this build.
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    Last edited by moleman; 08-27-2009 at 11:02 AM.

  2. #2
    aka: SDK1968 dutigaf's Avatar
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    that looks VERY nice on that front stub set up. nice start!
    say what you mean & mean what you say


  3. #3
    gunco irregular moleman's Avatar
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    Default barrel bushing

    Here's a drawing of the barrel bushing with the barrel jacket lugs. The actual specs may be a little different as I have changed some such as the indexing nub to prevent the insertion of an unmodified barrel. I got these measurements from the the largest dimension off of several demilled suomi receivers, figuring anything smaller was a result of wear. I basically started with a 5" section of 1-1/4"x.250" wall 4130 seamless tubing which has a .750" inside diameter. Turned both ends and turned down to 1.240" between centers which was pretty much as light of a pass that would take metal off all the way around the bushing. On my first build I left the lug area the full 1.250" and it was just starting to fit snug. The bushing diameter on my suomi receivers at the lug area is 1.235" on average. Since I know a 1.25" lug will fit I decided on the 1.240" so it would have some room to turn when dirty. I then turned the bushing down to the basic profile leaving the front area longer to cut out the indexing nub and making sure the section that goes inside the receiver tube is 1-2 thousands over the inside diameter of the receiver tube. A quick note of on the 1.040" diameter section between the receiver section and the lugs. I had measurements of between .395"-.4" for this width. I went with the .4" as any looseness in the barrel jacket fit can be eliminated by pushing the barrel bushing deeper into the receiver. If you make this area the .395" then make sure to lengthen the receiver section of the bushing to maintain the 4.3" overall length.
    The next step was to cut the barrel jacket lugs. If you look at a suomi barrel jacket lug it's .4" front to back. Top to bottem it is tapered smaller to the center. The bottem of the lug measures .4" but the top on the 4 receivers I checked measures anywhere from .440"-.460". The smaller measurements came from well worn receivers. The one I have that looked almost new had the .460" lug measurement. Since the bottem of the lug is .4" you have a wide range of what they could be and still work. I tried to get it as close to what the new receiver was at .460". I don't have a dividing head so I used the tape method to mark off the lugs. I took a piece of scotch tape and ran it around the lug collar, trimming it for width and cut through the overlaping ends (removing the extra scrap) to end up with a piece of tape that was exactly the size of the lug collar which I taped to a steel ruler with the end and a side matched to the ruler. There are 4 lugs each .460" so I multiplied that by 4 and subtracted that from the tape length. Then I divided what was left by 4 to get the spacing between the lugs. I then colored the tape black with a sharpie and alternated scratching lug or space lines with my dial calipers in the black sharpie on the tape. I then covered the first piece of tape with a second piece to keep the sharpie from smearing. I put the tape back on the barrel collar and used a piece of 1/2" angle iron as a guide to cut the tape with a utility knife through each scratch line. I removed the larger sections of tape leaving the smaller .4"x.460" lug tape. I then squared up the bushing on the milling machine with one "lug" on top. I ran a dial indicator across the top to make sure it ws totally flat after each time I moved the bushing. I cut the front of the lug collar until the 1/4" 4 flute end mill touched the tape. I then cut the back side of the lug until the lug was .460". Then I repositioned the bushing and repeated until I had the 4 lugs roughed out. I made one cut in the center of the "lug gap" to remove most of the extra material. I then locked a section of 3/4" rod in the vise with 5-6" sticking out and slid the bushing over the rod. Using an air cut off tool I rested one side of the cut off tool wheel cover on the 3/4" rod and ground between the lugs while I rotated the bushing lug to lug. When I though it looked good I colored the gap area in sharpie and tried to fit it in a barrel jacket. I reground any high spots until the bushing turned in smoothly.
    To cut the index nub I marked off a 1/4" nub centered on one of the lugs. I clamped the barrel bushing in the milling machine and ran the dial indicator around the top and adjusted the bushing until it was level. I then milled away everything that wasn't the indexing lug until I got close to .305" from lug to bushing front (not indexing nub). I would cut .001", bring the table all the way to one side and test fit a modified barrel and barrel jacket. When the barrel jacket would finally lock up easily I was at .300" after that I rounded the indexing nub with a file. The section that goes in the receiver is still oversized at this point and will need to get trimmed down for proper headspace, somewhere areound 4.24"-4.245".
    For some reason I keep getting an invalid file message for the bushing blueprint. I know its been over 25years since drafting class but I didn't think it was that bad. I'll try again in a bit.
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    Last edited by moleman; 08-27-2009 at 11:40 AM.

  4. #4
    gunco irregular moleman's Avatar
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    Quote Originally Posted by dutigaf View Post
    that looks VERY nice on that front stub set up. nice start!
    Thanks, I really like the suomi. My first one is a bit rough but I've relearned alot of what I forgot in machine shop from highschool over the past year so this one is turning out much better. I left the stock on my first one as I got it and it was by far the worst of the bunch, but I think it gives it that battle field pickup look.

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    gunco irregular moleman's Avatar
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    Default barrel and shroud mods

    The short suomi barrel needs to be lengthened and made so it will fit the modified barrel bushing, and the barrel shroud need to be modified for the longer length of the barrel. I first modified the barrel by chucking up the barrel stub and a live center on the muzzle end. The barrel has a slight taper to it so I turned the section just forward of the barrel collar a few thousands to remove the taper in that section. Then I turned the last inch of barrel down to .585 to match my barrel extension. I then turned the chamber end down to the same size as the inside diameter of the barrel bushing (.750). I fitted a section of 3/4"x.156" tube that I had previously drilled slightly over 1' deep with a 37/64" bit which made a .584" I.D. to the muzzle end for a press fit of .001". I then welded around the joint and cleaned the weld up with the lathe. I cut the extension end so that the barrel was now 16-1/4" long. I then enlarged the indexing slot in the barrel collar to 1/4" for the larger indexing nub.
    The barrel shrouds I have are the comp type. There is a little nub that looks like a muzzle in the front of the comp. I cut this nub off in the lathe and drilled/bored out the comp a couple thousands over the barrel size at the same spot. The barrel jacket now allows the longer barrel to slide through and lock into the barrel bushing. A quick note, I removed the front sight and used a center rest with the contact points running on the section that would be under the front sight so it would not leave a mark.
    Here's a pic of the muzzle being turned down for the barrel extension and a unmodified barrel shroud (upper left) and a barrel shroud modified for the 16" barrel on the lower left.
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    Last edited by moleman; 05-10-2009 at 07:14 PM. Reason: add measurements

  6. #6
    Grand Poobah Gunco's Avatar
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    Look at the mad skills on Moleman! Looking good.
    "Courage is being scared to death - and saddling up anyway." - The Duke

  7. #7
    gunco irregular moleman's Avatar
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    Thanks Gunco! I've actually got quite a bit more done on the build and took pictures along the way which I'll add on in the next few days. Hopefully it will make the suomi a little easier for people to build from scratch. They're a lot of fun to shoot and surprisingly accurate.

  8. #8
    Gunco Veteran Markp's Avatar
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    Moleman, awesome work... but I am still not getting how you cut the lugs... I feel like I am being terminally stupid... can you describe that process one more time... just not understanding how you cut the lugs and kept the lugs square and the rest was nice and round...

    Mark

  9. #9
    gunco irregular moleman's Avatar
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    Mark, I didn't take any pictures while I was cutting the lugs (I didn't think it would turn out good the first time). When I order more steel I'll cut a few more barrel bushings and take a picture at each step to better illustrate how its done. The first picture shows the bushing mocked up in the milling machine. You basically cut one side of the lug then the other and repeat until all the lugs are cut out (I'll go into greater detail when I cut the next one). This will leave you with a large ridge between each lug. I cut this mostly off which left me with two smaller ridges. To remove the ridges I slid the bushing on a 3/4" rod that was mounted in a vise. I then took an air cutoff tool and while keeping the tool stationary and resting one side of the air tool wheel housing on the 3/4" rod to keep it steady, turned the bushing on the rod and ground the high peaks off between each lug. The round area in front and behind each lug is the same size. I tried to remove as little material as possible and still make it rounded. The suomi receiver sections I have appear to have been milled in a similar fashion and have a slightly reduced diameter between the lug area. When you think you've got it, color the ground areas with a sharpie marker and try to slide the bushing in a barrel jacket. any high spots will have the marker rubbed off. Just hit the high spots with the grinder again and recolor/test fit and repeat until it slides in correctly. Hope this helps, like I said I'll take more pics of the next bushing and add to this thread. Heres some mock up pics of the lugs getting cut.
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  10. #10
    Gunco Veteran Markp's Avatar
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    Thanks man.... I can do that... I see how to do it! Like I said, great work!

    Mark

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