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.