M500-What Am I Missing?

[TaroBear]

So I picked up an ACM Mossberg M500 pistol grip the other week. I've finally gotten around to giving it a test. I gased it up with propane, and struggled through the (awkward) instructions and loaded it up with BBs. I squeezed the trigger and the gun shot...gas. The BBs are all still in the respective loading tubes, and they don't seem to be firing.

I suspect that the loading tubes must be lined up a certain way before the loading tube can be closed? On a side note, the little latch that holds the magazine tube closed can rotate almost 270 degrees...not just the 180 that the instruction manual suggests. Is there something I'm missing? Am I misaligning the ammo tubes somehow?

[Kos-Mos]

Here is a simplified version:

Flip the gun upside-down.
Unlock the magazine tubes with the latch. It can rotate a fair bit more than half a turn, but it does not mater.
Pull out the magazine tubes completely.
Fill each tube with 50BB. There should be a little space left when 50 are in. Don't pack it until it jams. It is a good idea to COUNT the BBs, as shooting with just 1 BB will fire it at over 450 fps.
When all tubes are full, push the magazine tubes back in.
Close the latch.
You should be able to rotate the tip of the magazine tubes as many times as you want. It will click in place when aligned. There is 3 "locked" clicks.
When the tube is aligned properly, cock the gun.
Push the cocking handle all the way forward until it locks in place.
Remove safety (top rear of the receiver, forward position is live).
Press trigger, it should fire 5 BBs. You have 10 shots from each tube.

There is not enough gas in the reservoir for all 30 shots, I was getting around 13-14 before I needed to fill again. An AI fill reservoir is a good thing to have on you with that gun. On-the field reload of the tubes is a BAD idea. The tubes are fragile and the part that holds the tube in place is also very brittle.

If you still don't have any BB shooting out, you will have to take it apart. I recommend you contact a gun doc, as it is VERY easy to break and replacement parts are nonexistent.

[TaroBear]

Originally Posted by Kos-Mos

Here is a simplified version:

Close the latch.
You should be able to rotate the tip of the magazine tubes as many times as you want. It will click in place when aligned. There is 3 "locked" clicks.

I think this might be the issue. The instruction manual was very vague about this. When I was done loading BBs, all I did was just slide the entire magazine rod back in and locked with the latch. I didn't do any of this clicking?

To clarify, am I rotating the entire rod before it goes back into the magazine tube, or while it's still exposed?

[Kos-Mos]

Once the brass tubes are inside the gun (inside what would be the magazine tube of a real shotgun) and the latch is locked, you can rotate them.

Basically, there are 3 "magazines" in the gun. You have to align one to the loading chamber in order to shoot.

When a mag is aligned, it should click.
It's just the little ribbed cap at the end (the part you pull on to pull the brass loading tubes) that you rotate. That in order rotates all the internal assembly. It should click when the tube is aligned.

[TaroBear]

Ahhh, okay. I'll try that out tomorrow. My mistake was that I assumed the brass tubes had to be aligned a certain way BEFORE they slid back into the gun, not after. Should I just gently rotate that cap until I hear a click?

[Kos-Mos]

Exactly. It should click in place when aligned. If not, the bearing that makes it click might not be there... if you bought new, contact seller. If you bought used, well good luck.

[Lessard]

Any idea on how to change out a fill valve on this beast? I have one, but it spews propane all over me when I try to fill it, I think it's a seal issue, but is there somewhere I can find a new valve? Or even one that I can jury rig?

[TaroBear]

Okay, just tried it out today. I managed to rotate the tube and fired off the first shot with BBs coming out. After that, just gas again. Would the thing that makes it click resemble a small set of O Rings, by any chance? I found a few lying on the bottom of the gun box today.

[Eeyore]

Originally Posted by TaroBear

Okay, just tried it out today. I managed to rotate the tube and fired off the first shot with BBs coming out. After that, just gas again. Would the thing that makes it click resemble a small set of O Rings, by any chance? I found a few lying on the bottom of the gun box today.

No, if bought new the gun comes with a spare set of o-rings.

[TaroBear]

Hmm. How audible is the click? I swear I heard it once, before I fired the first shot. Then nothing, even though I've rotated it to the max in both directions.

[Lessard]

It's pretty audible. There should also be a hole at the end of the magazine tube that you can see the bearing snap into.

[coach]

Rotate locking latch to open.
Slide the BB tubes out.
Locate a bearing near the front.
Locate corresponding hole in magazine tube.
Press bearing to see if the spring is still there.
Adjust spring tension as necessary. There is a grub screw accessed on the opposite side. Don't over tighten.

If the above fails, you got a lemon.

[TaroBear]

H'okay. So it looks like I can only get the first shot off. I sent it off to a buddy, he got the same result. The best we can figure out is that it seems the BBs aren't being loaded into the gun properly. It appears that the BBs are loaded into the gun via some kind of spring mechanism like AEG mags have, so I'm thinking it has a weak spring. Problem is, it looks like the entire tube is welded together, so I can't swap in a new spring to test. Any ideas?

[ap27]

Try holding the gun pointing straight up and then pumping it. That should negate the issue of a weak spring as the BBs should just fall into the chamber when the pump is pulled back and the chamber is rotated towards the feeding tubes.. If the BBs still don't load, you probably have an issue with the tube. There are quite a few guides on disassembling, with pictures.

[TaroBear]

Originally Posted by ap27

Try holding the gun pointing straight up and then pumping it. That should negate the issue of a weak spring as the BBs should just fall into the chamber when the pump is pulled back and the chamber is rotated towards the feeding tubes.. If the BBs still don't load, you probably have an issue with the tube. There are quite a few guides on disassembling, with pictures.

That gets mixed results...sometimes 4 shots can come out of the tubes, sometimes it's still one.