MGC68 barrel upgrade

Ordered some SRC parts to rebuild my broken Marushin mp41. When they came in, I found that I'd accidentally ordered a mp40 barrel. Since I had the parts I decided to upgrade my MGC mp40.

The barrel assembly of the MGC isn't one of the best parts of the replica. The barrel nut is very wrongly shaped. The front sight base and barrel are one piece cast zinc, and not the greatest looking. The sight hood is decent. Some are stamped steel, while others are zinc. The muzzle nut is steel, but pressed on.

The SRC barrel is cnc aluminum. The front sight base and resting bar are removable and made out of zinc. The sight hood is steel, but not as nice as the MGC. The muzzle nut is steel, but threaded m14lh.

The shank of the SRC barrel is the same size as the MGC so it looked doable to use it. First, I tried 3d printing some new half moons to fit the MGC barrel nut. The nut couldn't go back far enough.

Kind of wanted to replace the inaccurate nut. I had previously modeled a mp40 nut from drawings. I changed the internal dimensions to match the barrel and trunnion and printed it. After a couple of prints I got one that fit good.

Not really happy with a plastic barrel nut. Could order a SRC nut and machine it to fit the MGC trunnion. The threads of the MGC are bigger. The SRC nut is aluminum so it can't be blued with cold blue. Might make a new one out of steel.

Came out looking pretty close to a late war barrel assembly that I have. Much better than the original one.

The groove for the half moons is much farther forward on the SRC.

There's no indicating key on the SRC barrel. Once properly tightened down it will stay in place.

MGC trunnions come in different depths. Having a detonator also changes the depth. With my trunnion and detonator, the barrel sits 6.5mm forward of the rear stop. The barrel does sit at the depth it needs to be. The start of the taper lines up with the end of the barrel nut.

The front of the SRC trunnion is 6.25mm longer than the MGC. The SRC threads are smaller.

I don't have a SRC mp40 nut, so I don't know for sure if it can be modified to work. Might be able to bore it out and thread it to match the MGC trunnion. Some new half moons might need to be made.

The SRC sight hood does have some flaws. They can be fixed.

It's painted instead of blued. The easiest way to remove the paint is to burn it off with a propane torch. Then put it in an ultrasonic cleaner.

The hood is partially welded on the bottom. After welding the rest of the way and sanding it looks much better.

The top front edge is flat instead of being rounded from stamping. I turned a piece of round bar to 17mm. Then hammered it into the hood. My hood was a little out of round so I had to do a little tapping to get it round. Marked the angle and cut the round bar to match. Gave it a slight radius on the edge and put the hood back on. With a lot of little hits with a little hammer I rolled the front edge. A little sanding on the outside smoothed it out.

The bottom front edge is also flat. Rounded it a little with a file and some sanding.

The entire front is open. On another one I did weld in a piece. It wasn't easy because of the thinness. Wasn't completely happy with it so I'm skipping doing it on this one. Silver soldering the piece might work better.

Finished with some cold blue and oil. The little bit of work did improve the look.

After spending some time looking at the parts and measuring, I was able to use the SRC barrel with the MGC nut. There's only 1.5mm left of the half moons. Now that I have some working dimensions, I can make some steel half moons.

Made a spacer for the rear of the barrel. It's a snug fit and used Loctite 609 retaining compound to make it permanent.

Made a 3d printed jig to drill the barrel for a key. Used a m4 socket head cap screw with the head turned down to 6.2mm. Some Loctite 609 makes the screw permeant. Once set up the head was filed down until it fit in the trunnion.

New half moons were made out of steel.

Did have to take a little off the rear of the barrel to get the nut to tighten against the sling mount and washer. The printed half moons must have flexed enough to allow the nut to tighten.

Thought the barrel was painted. Turns out it was anodized. Some Easy Off oven cleaner removed the finish. Then it was polished with a blue Scotch Brite pad.

Used some Birchwood Casey aluminum black to blue the barrel. It didn't really work as well as I wanted. After several coats it did eventually all coat and even out. I only focused on the area that is visible.

The front sight has several marks from casting. On the left side there 5 indents from the ejector pins. The one under the hood isn't worth doing anything with. Alone the middle there is a parting line from the 2 haves of the mold. With some filing and sanding they can be cleaned up.

To strip the paint, I used some Easy Off oven cleaner. It took several applications to start to soften the paint. Once loosened it was scrubbed with a blue Scotch Brite pad.

With a diamond needle file, I draw filed all the casting marks. A little sanding smoothed out the scratch marks. Then the entire part was polished with a Scotch Brite pad. Finished off with some cold blue and oil.

Repaired a damaged barrel nut. Then turned it for a correctly shaped taper.
https://mp40modelguns.forumotion.net/t5923-barrel-nut-repair

Although the barrel assembly is made out of several material and finishes it does all match. Now I'm going to have to refinish the rest of the replica to match.

The paint was stripped off the resting bar. The parting lines filed and sanded. Painted with a low gloss high temperature paint. Baked for a couple hours. Oil was applied an allowed to soak into the paint. Then the excess wiped off.

Could have used the zinc resting bar. It looks good now that it's cleaned up and repainted. Early mp40's had aluminum resting bars so it does fit. I wanted a plastic resting bar.

3d printed the resting bar solid in abs. Drilled the pin hole. Sanded down all the layer lines. Finally wiped on some Pledge Klear floor polish to fill in the micro scratches and restore the shine.

Made a 3d printed barrel nut wrench. Since it's plastic it won't damage the flats of the nut. Still plenty strong enough to tighten or remove the nut.

Finally, the pins and muzzle nut retainer spring were installed. Then the barrel was tightened.

The barrel swap is now done. I'm extremely happy with the way it turned out. Looks really close to a real barrel assembly.

Great job, I would love to do that for mine