A long overdue work in progress.
Unfortunately you have to take down the entire gun like this because the charging arms are locked into the receiver during assembly.
Also I wanted to add another disclaimer; your shotgun is made by ACM I believe, so there may be differences from this Marushin. Just keep that in mind, and shoot me a PM if you have any questions.
And if you break it, it's not my fault.
1. Two philips screws hold on the the butt plate. Use a socket set to unscrew the bolt holding on the stock.
2. Remove the screw in front of the trigger guard and push out the pin holding the trigger assembly in.
3. The trigger assembly should come out easily. Pull down from the back first.
4. Removing the internals is the hardest part on take down and reassembly. Pull the pump all the way back. Slide the internals to the rear of the gun by pressing on the fill valve. Drop the front end of the internals down and the whole unit should come out (see red arrows in step 3). Clean out any grit and grease and re lube when reassembling.
5. Remove the tiny socket set screw from the bottom of the magazine tube. Pull the loading tube out. You can remove the pump at this point. Keep an eye out for the brass ring that may still be inside. It sits at the end of the loading tube to keep it aligned with the receiver. It is held in place by an o-ring on either side of it.
6. Remove two socket set screws from either side of the outer barrel where it meets the receiver. Holding the receiver, slide the barrel forward (it will be snug due to an o-ring inside). Be careful of the ring attached to the outer barrel; it is easy to crack.
7. Unscrew the ring in front of the pump handle and pull the handle forward to expose the pins holding in the charging arms. It takes a lot of force to move the plastic handle. Be patient.
8. Use an allen key to push the pins out.
9. Your charging arms are free! Replace with new ones here.
10. You can see why this arm breaks so easily.