I put a sgn-9 bolt up for sale in the classifieds http://www.gunco.net/forums/f259/wts...-9-bolt-47401/ to fund some other projects.
I've started a suomi build thread http://www.gunco.net/forums/f245/suomi-build-47460/ since this second build is going to be a suomi also. A lot of the techniques in building the suomi and the sgn-9 are similar and may be of some use to the sgn-9 builders.
Here's a drawing of a simplified suomi style front filler to use on a 1.25" tube for a sgn-9 type build. It doesn't have the takedown feature and is made out of 3/8" or possibly 1/4" plate (although feed ramp angles would be steeper). Its two sections that get welded together and welded to the receiver tube. I was going to use this design on my sgn-9 so that I could use suomi drums for their cool factor. The original rectangleish design that matthews uses in the sgn-9 article will not allow the use of the drums. I'm going to try to make a bushing for my sgn-9 that uses the lugs for a suomi takedown feature. I've made one for a reduced suomi receiver, and believe I could make one for the sgn-9 as well.
To avoid hijacking your for sale thread:
You said that the bolt #2 was around 21.5 oz, so i was wondering what the bolt stroke is, what spring youre using and if it is a floating bolt or if it hits the rear cap(if it only lightly strikes the rear cap, I dont mind but I dont like how mathews originally had to put a super heavy duty rear trunnion to keep the gun together). I am really wanting to build an sgn-9 and I have read deth502's build over on weaponsguild, and your build many times, but I still have some Q's
First of all I want to go with suomi mags because they are the same price as sten mags and are much more reliable. Also I want to be able to use drums and a coffin mag cause I think they are cool.
My question is about the magwell. This is comprised of the front and rear fillers? How do the mags lock in? Could these be fabbed out of some sheet steel bent and welded to shape or am I better off buying some. Lastly, the hammer slot that is milled in the bolt, will it fit an ar15 hammer or will it only work with the shotty fcg?
Desert fox, I can't answer all of your questions, but here's a shot at most of them.
#1. "You said that the bolt #2 was around 21.5 oz, so i was wondering what the bolt stroke is, what spring youre using and if it is a floating bolt or if it hits the rear cap(if it only lightly strikes the rear cap, I dont mind but I dont like how mathews originally had to put a super heavy duty rear trunnion to keep the gun together)."
The bolt stroke on the sgn-9 build I'm working on is 5" and uses a standard ar15 rifle length recoil spring. I can't tell you if its going to float or strike the end cap just yet. I plan on putting a rubber bumper on the back of the recoil spring guide rod like the ar15 buffer has just in case. With the 21.5 oz its pretty close to the sten bolt weight and recoil stroke. The sten isn't built especially stong in my opinion, but seems to hold up well. My suomi build that uses the 14.5 oz bolt with the older recoiling 2nd stage spring (2nd spring up from bottem) that brings its weight up to 16.5 oz (18 oz with the newer one center top photo) did impact on a rubber buffer. After more than 1000 rounds I noticed that the 1.2" long 2nd stage recoil spring had been compressed to slightly less than 1". It was rated at about 29lbs and I replaced it with a 1.6" long heavier spring rated at 59lbs. I made a collar to stop the spring from being over compressed which is what ruined the first one. The recoil stroke on my suomi is 2-1/4" on the original recoil spring until it hits the heavier spring. This is enough to pick up the next round from the mag. At that point it has impacted the 1/4" rubber buffer, but can continue to recoil another 7/8" on the 2nd stage 59lbs spring. It seems to work quite well and I've only made 1 rubber buffer and it is still in excellent shape showing little if any signs of wear. I started off with a 1/2" buffer, (bottem buffer in photo) but switched to a 1/4" one with the heavier 2nd stage recoil spring. With the first 29lbs 2nd stage recoil spring installed I could compress the recoil system the full length of the charging handle stroke. With this 59lbs spring it feels like the bolt hits a solid wall when it starts to compress. You can pull it back more, but you won't in actual use.
#2. "My question is about the magwell. This is comprised of the front and rear fillers? How do the mags lock in? Could these be fabbed out of some sheet steel bent and welded to shape or am I better off buying some."
The mag or drum is really only held into the gun by the mag catch holding the lug on the back of the mag or drum. The drums have a flat pad which contacts the front filler and rely on their width to keep from twisting, and the stick mags have a small lug that rides in a groove to keep from twisting. A few posts up I have a design for a simplified front filler that is made out of some 1/4" or 3/8" plate. The two pieces are simply welded together and then welded to the gun. If you weren't ever going to use the stick mags you wouldn't even have to cut a shallow slot in it for the front of the stick mags. Heres a picture of a suomi magwell. Its pretty open and you can see the mag catch that holds the mag in. I have thought out a simplified rear filler but havn't drawn it out yet.
#3. "Lastly, the hammer slot that is milled in the bolt, will it fit an ar15 hammer or will it only work with the shotty fcg?"
An ar15 fire control group works great and that is what deth uses as well as matthews. I just use the remington on my suomi-sgn-9 as its cheaper and fits in the suomi trigger guard easier (for me). I originally was going to use an ar15 fire control group on my sgn-9 build, but have scrapped that lower and have started on one that uses a complete remington trigger group. The hammer slot in the bolt can be up to 1/2" wide, I shoot for .395-.4" (10mm ish) which is the same as the rear sight base slot on the top of the suomi bolt. An ar15 hammer is about .3" wide so it will fit fine in that slot. I suppose for a remington hammer I would only have to cut a 1/4" slot or less for the .139" wide hammer.
Hope this helps, I'll try to get a drawing of a simplified rear filler that only requires cutting and welding up in a couple days.
Last edited by moleman; 06-07-2009 at 12:42 AM. Reason: add
Maybe he should have used a 1.225" id tube, then he wouldn't need to use a lathe on that bolt. He could have just milled a slot and used a blocking bar.
That's pretty slick looking... It would be far cooler as an SBR thought, unfortunately it would be like next year to get the damn ATF form back and they have publishing deadlines in the real world.
Markp, I looked for 1.225" id tube before settling on 1.5"x.188 wall tube with an id of 1.123" (I'm running 5 thousands bolt clearance). I've even tried using a boring bar to bore out the diameter to 1.225" but my tool post flexes way too much with the bit 10" out. Do you know where to find 1.5" od/1.225" id 4130 tube?
The problem is that the only metric metal place I know of doesn't do small orders. They had the proper tubing, I didn't call them because it was obvious that they supply in bulk.
Wicks and spruce aircraft supply have the 1.5"x.188 wall 4130 tubing. For the sgn-9 matthews went with the 1-1/4" tubing to keep the weight down I believe and so that he could use the ar15 style recoil system.