Hi all. I've always looked online for tutorials regarding the ACM M500 (clone of the Marushin M500 series) and never found a tutorial that directly explains what is needed to modify aftermarket charging arms to fit. Existing tutorials I have found show disassembly, but nothing more than that.

So, since I just got another pair of Deepfire M500 charging arms today in the mail, ordered from Ehobbyasia, I decided to take some pictures while I was doing the work. They are not the best because my hands were greasy and covered with metal bits while hoping not to scratch the screen on my phone!

This tutorial was done with an ACM M500 8mm version BUT also works with the 6mm. If you have a Marushin or different brand of charging arms, some things may be slightly different for you.

I did not have a dremel so I did the hard part with a metal filer...

BEFORE YOU START:

- I recommend you clear the gun/tube of BBs and shoot out remaining gas.

- For anyone who did not catch it, my rough tutorial involves filing the arms after reassembling the gun, which to some seems dumb to do in that order. If you're using a power tool that requires precision and safety, I recommend modding/filing the arms before reassembly.

In short, it is probably easier for you to do Step #6 (the filing/modding/cutting part) first.

- You will need this tutorial together with mine for a full disassembly and reassembly:

http://www.airsoftforum.com/board/ACM-M500...de-t155361.html


1. Comparison between the stock arms and new arms:

The new arms are full steel, while the stock arms are pot metal. Usually, the right-side arm breaks first because it constantly pushes and pulls against the mock bolt assembly. Some of the parts on the steel arms are thicker than on the pot metal arms, and some parts are also "bigger" which will affect the later part of installation.

The mostly-straight arms are the left-side arms., and the complex ones with the many curved portions are the right-side arms.






^ The last photos shows the section that give you some grief later.

2. Once you have disassembled most of the gun:

Don't forget to check these o-rings. These are 2 o-rings that should be kept in good condition as well, and they're not accessible normally unless you partially disassemble the gun.



^ the part connected to the inner barrel



^ the part sticking out of the loading mechanism. NOTE this picture is of an ACM M500 8mm version's thing. The ACM M500 6mm version has a similar looking part, and it's a bronze/copper color instead of silver/nickel/whatever.

3. Left-side arm...

When you first try to insert it into its channel, it will appear to get stuck. This is because it's thicker than the stock pot metal arms. So, I personally just pushed and pulled it back and forth until it friction filed it down just right to go through fairly smoothly.



4. Right-side arm...

This arm is the one that looks more complex and has the troublesome section we will encounter soon. This one slides in part of the way, and then gets stuck after a few inches. What I personally did was use a hammer to, gently, tap it through to let friction sand it down a bit. I would then pull it out, and repeat. This arm is trickier to file down.



^ note that I turned the gun's body over when I took this picture. Keep in mind which arm is the right-side and left-side

5. Getting the internals back in:

This is a bit tricky. The new arms' rear sections are slightly different than the rear sections of the stock pot metal arms, BUT in my experience they do not need to be modified in those sections to work with the gun.

WHILE YOU ARE HERE:

- You can check on the air outlet valve (it's like the "button" that the hammer hits to release the gas that propels your BBs). It has a few o-rings, one especially thick one, that may have dried out or gotten squished/ripped over time, or maybe the valve itself needs tightening. This is a common source of what sounds like a leak through the barrel.

Now, first put the piece back together. Fake dust cover / bolt thingy, then the fake magazine tube cover thingy:




^ other side. that loading lever thing is what spins the little circle under the dust cover

Tricky...before you start, have both charging arms approximately in these positions. I can't explain it too well in words, but basically for the right-side arm, the "U" area needs to be near the "cut-out" part of its channel, and rear tab of the left-side arm will be across from it in the same position. Also, look for a large hole. That's where the "fake magazine tube cover" (the long, funny shaped one from the side with the dust cover, not the loading lever from the other side!) thing's circular nub at the end will insert into.



You want to get the black lever's circle into that hole--if correct, it's "arm" will sit behind the end of the charging arm as well. After that, you need to get the dust cover's protruding piece to sit in that "U" area of the right-side arm. The lever thing from earlier needs to be somewhere between the left-side arms' two "tabs".



Pull the charging arms to their forward position. If correct, you might hear a distinct clicking noise. Now for the trigger assembly, cock back the hammer before reassembling.



6. Filing/Modding/Cutting time! (can and probably should be done first actually...)

First of all, don't forget to screw in the trigger assembly and put in its body pin. If every previous step was correct so far, you should be able to press the trigger to release the hammer (WARNING if there is still gas, please do this in a safe direction).

So if this is good, then you would just need to reassemble all the rest of the externals. However, now you will find you cannot get the handguard tube on all the way. The difference in front sections of the right-side arm will get in the way physically...





You might be wondering, "Why didn't you mod it before putting the main part back together??" That's because you don't need to do before then. I have done it twice, I know you don't need to, but it definitely might be easier for some people. The reason why I did not is because of the trial-and-error involved in getting just the right amount cut off.

However, I found out that while filing down the troublesome right-side arm, you can actually tuck in the left-side arm to give yourself more working space:



Now, here's what I did for my right-side ROUGHLY with a metal filer. If you have power tools like a dremel, I recommend that instead:



I did a small amount on the left-side just in case:





I used trial-and-error since I did this by hand, no power tools. After many tries, I finally got it to fit. With a dremel, you could cut the section and be done in a few minutes but I don't have one, so here was my way...


If you have any questions or comments, please feel free to discuss in this thread.

Currently, I have used the new arms on my 6mm version a ton, and no signs of wear. With this tutorial I have just finished it for my 8mm too, and it feels surprisingly smooth.

Remember the thickness of the new arms? One nice thing about not having filed it down for the channels is that no there's little to no wobble in the handguard. Previously when you have the stock pot metal arms, there is way too much side-to-side torque.