Question regarding Blowback mecanism / doubleshots

[MultipleParadox]

Hi,
I'd say I somewhat understand how the blowback mecasnism work in pistols (at least, for my KWA M93RII) but haven't completely wrapped my head around it, as I haven't really took the time to study the thing.

Currently my GBB is working fine except that it will do a double shot every 2 to 3 shots if the trigger is pulled quite fast;not double feeding but really two successive shots, in single shot mode of course. Burst/Auto mode seems to work fine but I don't use them that much.

I'm pretty sure this is caused by me changing the nozzle's return springs by elastic bands, that are either faster or slower to retract. This issue started happening since this mod. (why I did this is another story)

Now I'm wondering if there's a quick workaround to fix this double shot thing?
I thought that putting a spacer on the slide's spring to have it compress a bit more would help but it didn't seem to do anything.

Comment? Ideas? Stuff?

[Styrak]

Pretty sure your problem is related to the burst/auto feature on the M93R. Other GBB's don't really do "double shots".

[MultipleParadox]

Wouldn't the nozzle retracting too slowly or too fast affect this?
I'm thinking that if the slide complete its cycle faster than it should but the valve stays open while the cycle complete, it would trigger a second shot?

Last time I opened the grip to clean it out everything looked fine regarding the selector.
The problem really did appear with the switching of springs to rubber bands. And i don't have any spare springs to try switching them again

[AngelusNex]

something in the trigger mech is worn. I'm not sure what, but that's what causes this issue, that it to say unless it's just low gas double shots but those aren't 2 crisp shots like i gather your saying you get.

[MultipleParadox]

Yup
Crisp shots
I guess I'll take a look in the trigger/selector mecanism when I have the will to do so
I'll update the thread as needed then

Thanks for your help guys

[Kos-Mos]

The basic for a single shot in any GBB (Including rifles) is the same.

In the slide/bolt, you have 3 main parts:

Nozzle
Rocket valve
Carrier/BBU/bolt

Then the trigger mechanism has also 3 importan elements:

Hammer
Disconnector
Trigger

From Condition 1 (Loaded, round chambered)

The Trigger releases the Hammer. That lets some gaz into the nozzle.
The BB goes down the barrel with the pressure built behind it.
When the BB exits the barrel, it creates a void in the barrel, just in front of the rocket valve. The pressure behind the valve will be a lot higher than in the front because of that.
The Rocket valve closes the barrel side of the nozzle. That makes the gaz push fully against the BBU/Bolt and start the rearward motion of the slide.
About at half travel, the slide has enough speed and energy to complete the cycle. There is a nub that hits the disconnector at that point. It stops the hammer from releasing more gaz into the system. The slide also pulls the hammer back at cocked position.
Once the gaz pressure is off, the nozzle springs are strong enough to pull the nozzle back to the BBU.
Then when the slide starts to move forwards. The nozzle grabs a BB from the magazine and pushes it into the barrel. There is a notch in the slide when it is at rest that lets the trigger sear go in, making the pistol/rifle ready to fire.

In the case of a semi-fire, the trigger sear is made so that is slips off the release when fully at rear. You have to let the trigger go back to rest to reset the sears. In full-auto, the sear is not slipping, so as soon as the slide is back and ready, the hammer is released again.

Not if your gun fires two quick shots, I would guess that the sear is not released at the end of the travel, or the hammer is not locked when the slided is fully back. In both cases, the gun will fire as soon as the slide comes back. If you find that sometimes it will still fire after you let go of the trigger, it's the hammer that goes down.

[MultipleParadox]

Great explanation Kos-Mos, thank you!
There's a few details in there I didn't think about and it will definitely help me diagnose the problem.