Wednesday, March 16, 2011

Suomi M31 update, .50 BMG update

Well, this week is spring break for me and everyone in my household, so I have a little bit of time on my hands to do some building. I decided to go ahead and get a little work done on the .50 BMG bolt and get it ready for heat treatment, and to also finish up some things on the Suomi. I have to order some fire control parts for the Suomi to finish it anyway, so there's no rush on getting everything done.

Last time I posted, I had machined the receiver, threaded the barrel for screwing it into the chamber (to keep a factory short barrel from being easily dropped in and being charged with constructive possession of a SBR by the ATF), and had stretched the barrel to legal length. Now I needed to machine the chamber/barrel extension that would be welded to the outer receiver and have the barrel screwed into it.
I started off with a piece of 1.5" OD 4130 round solid stock. I needed a section roughly 5" long, so I chucked up a foot long section of it in the lathe, faced off the exposed end, and parted off a 5" piece. This piece was center bored in preparation for the barrel to thread in, then turned down to 1.125" OD for the first 4.5", and the last inch was turned down to 1.24" for the barrel sleeve lugs, with about 3/8" on the front side and 1/2" of the rear side of the lugs turned down to 1.00" to allow the barrel retaining sleeve to effortlessly twist into place once the lugs were cut.

Turning the piece down to size.

The rough turned piece with the larger section ready to have the lugs milled out of it. This part had to fit exact, so I rough turned it to within .002 and then sanded and polished it until it fit perfectly inside the 1.125" ID receiver.

Now, the part needed to be completely drilled out for the barrel threads, so I got to the arduous task of boring the whole thing out to 11/16".

The part was drilled out in increments of 1/8" until I got to 5/8", then I switched to an11/16" cobalt drill bit I bought from Fastenal. I have never purchased better drill bits than the cobalt ones I get from Fastenal. My cobalt drill bit index from there was about 150 bucks, and this 11/16" drill bit by itself was about 75 bucks, but they will cut through just about anything and stay sharp forever.

As you can see in this next picture, I have already cut the lugs on the mill. I was so preoccupied with cutting them right I forgot to take a picture. Suffice to say that what I did was to take the old shroud lug assembly and tacked it on the end of this piece I'm machining. I then was able to line up my end mill with the lugs on the old piece, and simply turn on the mill and move the mill table straight back to cut the lugs in exact alignment with the old ones. I wish I had taken pictures, because it sounds weirder than it really was. With the endmill depth set, I would rotate the piece a few degrees and mill off the excess between lugs. Took about 30 minutes to do.

Here we are running the 3/4"x16 tpi tap into the machined piece. I hate manual threading, by the way. This took a while to get threaded to the right depth.

The completed section with the now-lengthened barrel threaded into it.

Here's the next challenge to solve: the barrel shroud. Since the barrel is now 16.25" long, the old barrel shroud no longer fits.
Note that I had to cut the welds on the front mag piece so that I could eventually weld in the barrel extension.

Well, I can't imagine running around the range (or anywhere else, for that matter!) with no barrel shroud, so had to either:
1. Drill out the end of the shroud/compensator and let the barrel protrude (which is what TNW or whoever it is that makes these commercially does it),
2. Stretch the compensator, which wouldn't have looked right to me, though it might to some...
3. Stretch the actual barrel shroud, which is what I chose to do.

Now, I have seen a few of these rifles built wherein the shroud was stretched, but the builder used a normal piece of tubing behind the front sight, so there was a 4" long blank area to weld in place. I didn't really want to do this. I liked the slots that the shroud has from the factory, and wanted to continue the slotted look for the extended area. The slots themselves are only about 2.5" long, and I needed about 4" or so more room. I didn't really want to make the whole thing look too long, so what I did was turn the muzzle end of the barrel down just slightly where it would fit inside the compensator on the end of the barrel, and then welded in a small section of barrel sleeve from another sleeve I had laying there. This gave me the ~4" I needed while only having to add about 2.75 inches from the other barrel sleeve.

First up, parting off the barrel sleeve that will go on the gun. I parted it off in the lathe so the cuts would be perfectly straight and also so I could cut slight bevels on the edges at the same time to allow for my welds to sink deeper when re-welded.

Installed on the gun to get an idea of how much stretching I needed. The shroud is just loosely mounted on the gun, so it was not perfectly on-center. That is why the barrel does not look centered here.

Anyway, I dug out another barrel shroud and cut a slotted section out of it by parting it off on the lathe. After a little sanding and cleaning, I welded everything together, then turned the welds down on the lathe as well. Kicking the lathe into high gear and hitting the shroud with 100 grit sandpaper and then scotchbrite gave me a decent polished surface.

It's a little weird looking at the moment, since there's no mag or magwell in place to go by. However, if you see that small tab hanging down from the bottom of the barrel sleeve, that is roughly where the end of the magwell is, so the barrel shouldn't look quite so long once that's in place.

There is a bit more work that got done on the Suomi, but I need to upload the pictures first.

Now, I finally got tired of not having a .50 BMG around to shoot, so I decided to machine the bolt to take an extractor. This made me pretty nervous, because if I bugger it up, I will have to machine ANOTHER bolt (number 3 or 4, can't remember which), so I handled this with kid gloves.

Getting the bolt ready to mill was the first task at hand. No, I did not use that large end mill to cut out for the extractor. I had been machining something else and had a few minutes to spare, and decided to dive in to machining my bolt and hadn't switched end mills yet before taking this picture.

I machined the extractor slot real quick with a 3/8" endmill, then disassembled the bolt into it's individual pieces. I haven't cut the extractor yet, I hope to do that tomorrow.


So, there's some progress on both rifles. I actually got a lot more work done on the Suomi last night, but will post on that later when I get the pics uploaded.

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