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Suomi build

18K views 78 replies 18 participants last post by  BIG 54R 
#1 · (Edited)
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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#39 ·
Its not perfect as I've found differences between actual front fillers and what is given in this drawing of an 80% front filler. They're close enough to make a functioning filler out of it though. There is or was a guy on weaponsguild that was selling front hillers he makes.
 

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#46 ·
Hello barrel guy,
If you can't find a useful repair section I'd weld in the ruined area of the threads using an inside aluminum plug as a chill block mandrel. Use plain steel filler or also a piece of plain steel to fill in the missing area.

That aluminum plug is a press fit mandrel and should also serve as a lathe center to shape and re-thread the new material via single pointing. Pick-up the original (metric ??) thread pitch and continue it into the repair area at only perhaps 5 thou initial depth of cut lest it blow the tip off your threading bit. Hard spots resulting from repair welding are best tackled with a carbide threading insert tool.

Being a barrel mfg you will know lathe single point threading very well. This technique will help repair threads on other torched items. Happy machining !!

VD
 
#47 ·
The last .4" or so of the receiver is threaded 20TPI at around a 1.455" outside diameter. Most of the threads on this gun are standard for some reason or the metric equivalent. The trigger guard screw is a #10-24 IIRC.
 
#49 ·
trigger group part #3

After converting a few suomi trigger guards to use the remington FCG, I've thought of a few ways to make the trigger group parts a little better. The main parts of the conversion are a front and rear block to hold the hammer, sear, disconnector, and hammer spring in alignment. I had previously used a .090" spacer on the left side of hammer pivot pin and a .080" disconnector on the other side to keep the hammer more or less centered. I changed the front trigger guard block to have a .090" shelf to act as a spacer where as before I milled it away. This will also give a little extra meat to hold the hammer pin. On the rear block I raised the center section up .085" and then milled out a .085" deep pocket for the sear to sit in. This was to keep the sear better aligned and to make the rear block stronger. In the pic the new style front block doesn't have the rear filler lug machined into it yet, and the rear block doesn't have the upper right fork trimmed for the disconnector. A original suomi front block is shown at the top for comparison. The blocks are made out of 1018 steel and I case harden mine, even though untreated is probably fine as remington now uses plastic for some of their trigger housings and the originals are aluminum. I have a couple that weren't heat treated and they seem fine. I machined these out of a 3/8" thick bar, but a 5/16" cold rolled bar is only a few thousands under and can also be used.
 

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#50 ·
trigger group part #4

Here is some more pics of the trigger groups blocks ready to be riveted into the suomi trigger guard. Also shown are the trigger bar and mounting block to the trigger. The original suomi safety is modified by cutting it at the first detent groove to shorten the upper bar. Then it is installed along with a bb and ar15 disconnector spring that fit into a recess in the back hump of the rear trigger guard block. Work the safety back and forth about 100 times so the bb makes a rub mark on the top of the safety. Remove every thing and make a small dimple at each end of the wear mark with a 3/16" diamond ball dremmel bit. You just have to make a slight dimple, about the size of a pencil lead to get the safety to snap into fire and safe. Any more and the safety will be hard to work. The bottem photo shows the safety in the fire position. In the safe position the trigger is prevent from being pulled because the upper rear section of the safety blocks the trigger via the tab sticking down on the trigger bar mounting adaptor The upper rear portion of the sear where the original coil spring would rest needs to be trimmed until the sear no longer sticks past the back of the trigger guard. Make sure it is trimmed well enough so that in the fire position when the sear is the to the rear it will not touch the charging handle block. Then make a groove or lip to retain the double torsion sear spring.
 

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#51 ·
mag release hole location

Can anyone give me the location for the pin hole that holds in the mag release lever? I know it's like a 1.000 down from the top of the rear filler (got a brand new one) but how far back from the front edge (mag well side of the filler)? I want to put it in the right spot so it holds the mag in and releases correctly.

Thanks!
 
#52 ·
I've measured several demilled rear fillers with intact mag lever holes. I got .370" (on center) back from the face (not the recessed part of the face though), and from the top of the mag release channel down I get 1.035" (on center).
 
#55 ·
Really nice work. You are some craftsman. If I remember the Suomi has the ejector welded inside the tube. I'd like to see that! Along with the ejection port and mag well. My build was ruined trying to get the drum holders just under the bushing. I'd like to try again. Moleman, could you make a tube with the bushing and threads for the end cap. I'd ask you to do the ejector but that might be over the eighty percent factor. Best, Joel
 
#56 · (Edited)
Joel1000, Put some pics up of how the build went wrong. Perhaps someone could figure out a way to save it. I have gone about putting an ejector in the suomi two ways so far. In either way you make a slot in the barrel bushing, (centered on the left barrel jacket lug) where the ejector needs to be. From there I've welded in an ejector before I pressed in the barrel bushing into the receiver. The only problem I've had with that is it is tricky to machine the mag/ejection port openings and not machine the ejector. The good part about it is the rear sight guides the bolt. If you have the rear sight slot done and insert the barrel bushing with the ejector installed you can rotate the bushing into position as you press it in to line up with the ejector slot in the bolt. The latest way I've tried is just mill the ejector slot 3/16" wide. Just press in the bushing so that the ejector slot on the bolt is more or less centered with the barrel bushing ejector slot. Once you have the barrel bushing welded in and the ejection and mag openings done, drill two holes at the front and back of the barrel bushing slot through the receiver from the ejection port. Then either make a screw on ejector or weld/rivet on ejector by taking a prefitted 3/16" thick bar and coloring the back side of it with a sharpie marker. slide the bolt back and forth lightly a few times so that the bolt impacts the oversized ejector. Mill away any part of the ejector profile that isn't still colored by the sharpie marker. Check for fit and adjust if needed. I've been busy with other commitments but hope to start posting some updates to this thread soon. I'm not comfortable making the receiver parts for others without an atf approval letter, and I don't plan on sending in a sample to get one. Especially since it wouldn't be a stretch for the barrel bushing to be considered a receiver since it can accept a bolt, barrel and barrel jacket. Prexis is probably going to be your best bet on an 80% receiver. Here is how I've been cutting the magwell opening so they are about 45 degrees by squaring the receiver up in the mill and placing a level on two of the barrel jacket lugs and adjusting until the level is flat. Mill one side, rotate then mill the other. The magwell opening doesn't come out tapered front to back like a suomi magwell, but it's a good compromise. I'll try to document this better than I have other builds. I'm good at building, not so much actually remembering to stop and take a picture.
 

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#57 ·
I kinda did what Moleman did but just a little differently. After my tube was all together and etc...I measured the cut receiver for the ejector location. I milled a .125 slot. Towards the outer edge of my receiver tube I opened up the slot to .187 about half the distance. I took the scrap receiver (ejector wasn't damaged) and milled it out. I machined the outer edge of the ejector to .187. Hand fitted it to the slot in my new tube so it was a snug fit. Put it in and it lined up perfectly with the bolt. When I'm ready to I will have it welded in place. I hope this made sense.
 
#58 ·
Barrel guy, I've thought about doing that also, but figured it would be hard to do since the original ejector is riveted into the suomi receiver. Glad to hear that way is an option also as I have a few receivers that have intact ejectors.
 
#59 ·
I have only been able to find 1.5" 4130 tubing in .120" wall thickness (too big I.D.) and .188" wall which works but makes for a heavy receiver. I recently found .156" wall thickness (listed as 5/32") at Factory Steel- 4130 Alloy Tubing, Aircraft Bars, Tubes in Square, Round, Rectangular Shapes which give you an 1.188" inside diameter. They will sell by the foot so you don't have to buy a 10' section. A 1.188" i.d. is the same or darn close to the comercial reduced diameter receivers out there. Does anyone knows a source for 1.5"x.138"-.140"wall 4130 tubing for original i.d. (with a blocking bar of course) receiver tubes?
 
#61 ·
Thanks barrel buy. I've tried wicks and spruce aircraft, online metals and speedy metals and a few others without luck. I've searched on line several times but never seem to find any.
 
#63 ·
If its metric then the outside is close enough to 37mm o.d. with a 3mm wall thickness. For the reduced tube then it would be 37mm o.d. with a 3.5mm wall thickness.
 
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