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I recently bought a JG M4 S-System and after noticing erratic shots with Elite Force and Golden Ball BBs, I did some searching across various Airsoft forums. All of the post I read stated that the stock bucking on the JG M4 S-System is crap and to replace it ASAP. So I ordered a Systema bucking and following the guides on Mechbox.com and a few youtube videos I replaced the bucking. The gun went back together like a dream and nothing got in a bind. The gun will now fire 3-6 BBs at once (EG: on one shot, not one trigger pull.) There are multiple BBs flying out the barrel at once and hitting each other.

I pulled the barrel again and checked the bucking for damage, and noticed nothing. I also saw no damage to the upper hop up or the lower feed. I did not replace the nub however as the systema did not come with one.

Can someone please enlighten me on what could be causing this or what is most likely wrong. I am a very mechanically inclined person and have many years in the DIY / Maker scene so I am comfortable tearing down the whole gun if needed. If you need any photos, or a better description please let me know.

Thanks in advance! CJ
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You probably don't have the bucking seated in the hop up chamber properly, and it's not stopping bbs like it's supposed to. A second problem may be that the hop up isn't being pressed up against the gearbox like it's supposed to be. Do you still have that little spring on the hop up that presses it back against the gearbox shell?

Yes the spring is still there and the bucking appears to be properly seated with the little line seated in the groove on the barrel and the bump square in the cutout. Someone mentioned that the Nub may be out of alignment.

Unlikely. Even if it was misaligned, or out of place, it would only effect the hop, not the loading of bbs.

The bucking may be bunched up in the hop up, while it looks like it's properly adjusted, it might still not be seated all the way forward into the gun. Try adjusting it a few more times to see if you can't get it to work properly. Chances are you'll get it right simply by taking it apart and putting it back together a few times.

Thanks! I will try that tomorrow after work.

Spent about an hour reseating the bucking and hopup a few times. It seemed to fix the issue for a few minutes on the first try then after removing the mag and placing it back in, the multiple feed issue was back.

Please note that I am not an expert like star but I have had this same issue with the same gun and it sounds like the bucking is not seated correctly like star said and from my experience you should try to start all over again. Take the hop up chamber off take the bucking off and try again. You may have just gotten unlucky and when you put the hop up assembly back on over the bucking it may have pushed it out of position.

I have removed and reinstalled it about 11 times now. I do not see any damage to the bucking, hopup, feeder or the air nozzle so I am ruling out damage. I will try reseating the bucking again tonight. Just to clarify things, the raised line inside the bucking slides in the groove on the barrel and the little bump fits in the cut out hole on the barrel. I am quite sure its not bunching up and is seated as far in the hopup chamber as it possibly can. It will not slide in any further and looks flush with the raised portion at the back of the hopup unit.

I don't kow much, and mayor may otbe bashedfor posting this, but could it be his mags?

Haveyou tried a different magazine?

Well I'm assuming that he is using mags he already knows work but it still could be you mags. Good point.

If you are sure nothing is damaged or misaligned, then it must be the one of two things. Either the bucking isn't sitting correctly in the hop up chamber, or, your mag isn't feeding properly. Given that your mag fed just fine with the old bucking, it must mean your bucking isn't in properly, or isn't seated properly.

I know it seems repetitive, but just because the bucking is sitting properly on the barrel does not mean that it's still sitting properly on the barrel once you fit it into the hop up chamber. You man need to use some silicon oil on the outside of the bucking to make sure it slides into the hop up chamber properly. You may need to twist it back and forth a bit to get it to fit as it should.

I fixed the problem. I guess the FPS of this JG was just too high for the systema bucking I had. I replaced it with the stock one and it fires fine with out a single double feed. I swapped it for the systema one more time and the problem was back. So I swapped back to the stock bucking and all is fine.

While I was in the gun I cleaned the gearbox and relubed it with white lithium grease. Then replaced the stock spade connectors between the Gearbox and S-System Battery harness with 3.5mm bullet connectors. I also rebuilt the S-System battery harness with 16awg silicone wire and a deans connector. This made installing the battery 10000 times easier.

My Turnigy Nanotech LiPo's will be here Friday just in time for Blue Hole on Saturday! Making a trip up to Airsoft Atlanta tomorrow to pick up a gun for my brother in-law as well as a Element HARD (65) blue bucking in hopes that it will fix the feeding issue the systema was causing.