IT sounds like you've jusy blown your piston head or you've cracked or broken something inside your BBU. Take it apart and inspect every part carefully. Cracks are easy to miss in poor lighting.
But to be honest, I'm not surprised that this came up. My two WE Glocks have the same problem. Both a G17 and a G19.
The problem you are experiencing is that there is too much wasted gas. There no real "blowback" to the gun because simply put, gas can't even make it into the upper assembly.. or at least not enough of it. This is by by WE's design of the slide itself. The BBU is too embedded within the slide that there is a lot of room between the magazine and the loading muzzle, thereby giving the gas expelled from the magazine more room to disperse while it is on its way to the loading muzzle.
The second problem I've noticed is that the loading muzzle itself doesn't fully return to it's resting position and thereby misaligning the whole in which gas enters the BBU on the loading muzzle.
Gas efficiency sucks on these things because whatever pot metal mixture WE used for the slide is too heavy. Thus more gas is needed to move it and complete a full cycle.
These are both issues found in both my WE Glocks. Am starting to think that this may be the case with their whole Glock line in general. But I could just have gotten the bad ones out of the whole production line. Who knows. These are hard problems to fix because they require a lot of playing around with the internals.
but I'll try to give you a few POSSIBLE solutions and what I did to get them working properly on MY gun.
This is going to get confusing so bare with me while reading.
The BBU and it's internals explained:
The whole blowback assembly's function is two folds: 1). To provide the bb with adequate propulsion and 2). To reset the valve knocker while simultaneously chambering the next round. so here is each part and what they do:
The Loading muzzle (part g26-6):
The loading muzzle's function is two folds: 1) to push the bb out of the barrel and chamber the next round and 2) to create blowback so that the slide can reset the hammer to the ready fire position.
The arm on the loading muzzle chambers the next round by pushing the next BB out of the feedlips (part g26-62) on the magazine. The whole in the front directs air that pushes the bb. the bottom whole is where air passes through from the magazine.
When the gun is fired, the loading muzzle tends to stick to the hop up chamber until the whole slide pulls it away. There is a spring located atop the loading muzzle that returns it to it's resting position when the "blowback" occurs. This is a very important part as it may contribute to the problem at hand by not fully returning the muzzle and causing the misalignment mentioned above.
The Floating valve and plunger spring (Part G26-8 and Part G26-7):
This is located inside the loading muzzle and is the heart of the BBU. The function of the floating valve is to distribute the air from the magazine to push the bb out and to create blowback.
You'll notice that the floating valve has several holes cut into it and down the middle. This is to direct air towards the BB.
Understand that much more force is needed to move a metal assembly and all its internal parts (the slide) than a 0.20 gram piece of spherical plastic.
At some point during the cycle, the floating valve will completely close off the front part of the loading muzzle by the negative pressures in created when the BB travels and exits the inner barrel, cutting off airflow to that section and redirecting the remaining air to the rear of the BBU.
The plunger spring returns the floating valve to it's original position, ready for the next trigger pull.
The Piston Head (part G17-4 and Part g175):
The piston head prevents or minimizes the amount of air that escapes the loading muzzle on the rear side of the BBU. Essentially, without the piston head, you can't achieve "gas compression" since all the air escapes. Know that air will tend to escape which ever way it can.
The hop Up bucking (Parts: G26-24):
This acts exactly like a piston head except it's found on the front of the upper assembly. It also adjusts the "hop" on the BB by making the BB spin making it easier for it to splice through the air.
That's all that really matters for your question.
The hammer bearing's function, again, is two folds: 1) to lock the hammer to the sear and 2), provide a "cushion" to the BBU when the hammer is released.
Understand, that your hammer doesn't actually hit the BBU as hard. That's why, when compared to the latter KSC models that DO hit the BBU, the Marui's hammer doesn't have much wear. This is all possible because of that hammer bearing.
The BBU is also not the main culprit when the hammer is reset. The Hammer bearing is. Here's a cool experiment: try firing the gun without the hammer bearing and see if it does reset and lock the hammer to sear.
The sear is the main contact point between the front of the gun to rear. Without it you can't fire the gun. And yes the sear does lock to the hammer. The purpose of this is stop the valve knocker from hitting the blowoff valve on the magazine over and over again whether you want it to or not.
An enhanced hammer spring creates a higher velocity of the hammer when the sear releases it. This in turn creates a much more powerful strike from the valve knocker to the blowoff valve on the magazine. This opens the blowoff valve much more and allows more air to escape from the magazine.
The enahnced hammer spring also a tendency to create more resistance to the slide when it is cycling. This is because the enhanced hammer spring has a higher tension.
So how does this all come together to create "a full cycle"?
This happens in less than a second or two so the details may be a bit shotty at best. But in general, this is what happens:
1). First, the gun is racked and the hammer is in a ready fire state. The valve knocker is ready to hit the valve on the magazine.
2). The trigger is pulled and the trigger transfer bar pushes the sear back and releases the hammer.
3). the Hammer, along with the valve knocker, dart forward and hits the valve on the magazine, releasing gas.
4). Gas is expelled from the magazine and enters the BBU via the bottom hole of the loading muzzle.
5A). The BB begins its travel down the inner barrel.
5B). The BB exits the barrel. The floating valve is closed due to negative pressure, The piston head and the head of the floating valve create the necessary gas compression long enough to "blow" the whole slide assembly backwards. The slide begins it's travel backwards. (Some may disagree with 6A and 6b occuring in different times. But in general, it's what happens)
6). The slide continues its travel backwards.
7). As the slide is on its path, it trips the disconnector and the trigger transfer bar but doesn't reset the valve knocker nor the hammer to a ready fire position.
8). The hammer bearing and the hammer are pushed backed.
9). Just before the slide comes to a stop, the hammer is push down completely and the sear latches and locks it into a ready fire state through the hammer bearing. The valve knocker is reset.
10). The recoil spring pulls the whole slide assembley forward.
11). As the slides travels forward, the arm on the loading muzzle pushes the next bb into the hop up unit, chambering the next round.
12). The slide returns to full battery and the gun is ready for the next trigger pull.
Possible solutions for your problem:
As far as my tests go, I can only surmise the problem occurs somewhere between steps 4 to 6B. SO here are a few suggestions to consider:
Option 1:
While it may be true that the problem is not with the magazine. Do not be so quick to rule it out.
Even though gas is released consistently from whatever magazine you put in there, you have to wonder where the hell all that gas goes.
Alot of the times, a gun loses air compression solely for the fact that the magazine is not sitting correctly inside the magazine chamber. This causes a misalignment with the bottom whole of the loading muzzle.
Don't you wonder why the gun doesn't even move a single centimeter when the gun vents out all the gas with one trigger pull? In theory it should, right?That's because the gas is not directed into the loading muzzle at all.
There is either too much space between the magazine and the loading muzzle thus gas escapes everywhere before it reaches the loading muzzle or the loading muzzle itselft is not fully returning to where it should be (remember the spring I mentioned? yeah that guy...). This is all happening in step 4 above.
the fix is to get a new magazine catch to sit the magazine properly.
or you can do this:
You can bring your BBU closer to the magazine rubber whole thingy (the rectangular whole atop the mag) by putting a small amount of duct tape inside your slide, right above where the top of BBU will sit. You bring the bbu closer to the magazing rubber gas route thereby eliminating the uneccessary gas discharge and directing the gas directly into the Nozzle. dont put too much tape though. if you do, you cause the bbbu to be too close to the magazine rubber and wear it down even further. And also, the guide rails' path will be blocked by the bbu and the slide wont cycle at all. also the loading arm on the nozzle will jam the gun if there is too much tape.
You also don't want to put too much because duct tape is very malleable. When you tighten the rear screw to bolt the BBU to the slide, the pressure it creates is exacted upon the rest of the BBU and therefore cause too much friction on the loading muzzle. The BBU housing is not a closed off area like the KSC Glock BBU where this method wont affect the loading muzzle as much. In fact, if you put too much, the duct tape will actually take the shape of the grooves on the BBU housing and the loading muzzle won't move at all...I hope you know what I mean. The harder way is to put the duct tape where the rear screw is so you ont have to worry about the muzzle as much.
Essentially, if you are familliar with the shimming process of an AEG Gearbox, it's almost the same effect and process.
Or you can take apart your magazines and bring the air nozzle closer to the BBU. Same effect. Just a different part of the gun.
Option 2:
Is to check everything inside your BBU. Make sure nothing is cracked and nothing is broken.
My suggestion is to get a good piston head to get a better airseal inside your BBU. This will help conserve however much gas is in the BBU and effectively creating a good, crisp cycle.
Option 3:
Sand or file down the interaction point of the hammer bearing on the BBU (where the hammer bearing appears to be catching) and smoothen it out with polish or something.
Dn't go too far though. If you take too much off, the hammer bearing will become useless.
Option 4:
Buy a smaller hammer bearing. I have a 8mm bearing installed on mine and I've never had any iissues at all.
You gun jammed at the end because there wasn't enough blowback to pull the loading muzzle apart from the hop up unit. Once you create more gas flow, it will seldom happen.
Also, the gun vents all the gas because the slide doesn't travel back enough to reset the hammer and valve knocker. It's kind of an oxymoron or a paradox. This means that valve knocker will stay pushing unto the blowoff valve until either the mag runs out or you manually cocked the gun.
I can also be that your magazine is just cold....
I apologize if I can't help you further. It's really hard to diagnose without the gun in front of me.
Anyways, hope it somewhat helps.