Disabling blowback on a Hi-capa 5.1 GBB

[spazzytroy]

**This is a repost of my thread at airsoftretreat.com. I'm hoping to borrow the brains of some of the custom pistol makers here(coughthe4thpower3 coughIllusion) who otherwise might not see this thread. It seems like people here might be more apt to take a crack at custom modifying internals. If this is a problem just let me know and I'll take it to PM**

The ultimate goal of this project would be to replicate, on demand, the same kind of behavior that you get when you fire that very last bit of gas out of your gbb. You know, where the pistol fires the bb, but doesn't have enough gas to cycle. I realize this requires manual cycling after the shot, but that's what I'm going for. I'm not sure anyone has tried this before, so I figured I'd ask before reinventing the wheel.

Currently 3 goals need to be met for me to consider this a success:
-A decent velocity in "repeating" mode
-Little to no movement in the slide, and the blow back unit staying sealed against the hop rubber
-The ability to enable/disable this feature quickly, most likely with an extra thumb lever around the slide release being held down to activate.

The first thing I tried to do was just pin the slide forward with an action lock. This had the effect of firing one bb and then dumping gas into the atmosphere where the mag meets the BBU. I later figured out that the part that holds down the valve isn't disengaged until the slide starts to cycle. Despite the airsoft solution not working, this is how the RS version works so at very least I figured out how to stop the slide from moving. See Vietnam era MK22 mod 0 (Of note a modified S&W M59, not a 1911. The action lock is thing that looks like a really long slide release).


Next, I tried to hold down the valve knocker locker (I think that is what it's called) by inserting a shim into the bbu underside to hold it down the entire time. This is much like the way people are short stroking hicapa 5.1s to make Xtremes, and causes the valve knocker to be retracted early.

(Thanks the4thpower3)
That was much like mine except mine took up the whole space.
So far, it hasn't worked, though I'm sill trying to make sure it seats correctly. In addition I'm not sure if the valve knocker locker can be deactivated before the trigger reaches half cocked.

If someone has a different approach, like through a trigger mech mod, I'd love to hear it. If I can get this to work I'll have the ability to put out very very quiet single shots on targets too close for my vsr. Then if I miss I have the option of quietly cocking it again for a second shot or racking the slide for some semi action. Maybe in the future I can mix this with the full auto 5.1 trigger mod to have the ultimate sidearm. Long barrel? Check. Compact? Check. Deathly silent when need be? Check. Full auto when need be? Check. I'd pack one.

Thanks in advance for any help

Just some stuff I've collected dealing with mods I've discussed:
US Navy prototype - MK22 pistol? - THR
https://www.airsoftcanada.com/showth...t=72089&page=5

EDIT: I don't want to do NBB. I like the single action trigger pull much better. And besides, you can't modify NBB to full auto to my knowledge.

[ShelledPants]

You'd have to manually cock it each time to load the next bb and cock the hammer, but wouldn't putting a really stiff recoil spring cause the gun to operate properly (fire one shot at normal fps), and then prevent the slide from moving back?

[Endymion]

Not allowing the slide to move at all would cause the mag to vent all of its gas - there's a knocker release lever that is actuated upon the slide cycling back.

[ILLusion]

An interesting idea. I won't be the one to say this is an impossible task, but I'll go so far as to say that the NBB design and GBB design are so vastly different, that I personally can not conceptualize a hybrid design without redesigning the wheel and rebuilding all of the internals and frame structure from the ground up to support such a demand.

[spazzytroy]

Originally Posted by ShelledPants

You'd have to manually cock it each time to load the next bb and cock the hammer, but wouldn't putting a really stiff recoil spring cause the gun to operate properly (fire one shot at normal fps), and then prevent the slide from moving back?

Endymion is right. Like I said when I first attempted the action lock, you cannot simply hold the slide. Also an recoil spring would not allow the simple on/off of the action lock.

I'm ok with having to manually cock it. The purpose of having the gun not cycle is that it vents all gasses through the suppressor, making a very very quiet shot, instead of leaking them through the breech.

EDIT: illusion: do you think im on the right track trying to pin down the valve knocker locker?

[MadMax]

first step, put some tape on the firing pin lock to hold it down. That'll prevent the firing pin from locking forward and venting enough gas to cycle the slide. If your gun chronys consistently well with the firing pin lock disabled, then you're pretty much halfway there. I would think that your lockout would most elegantly work by disabling the firing pin lock which would both prevent blowback and mag dump.

[ILLusion]

Originally Posted by spazzytroy

do you think im on the right track trying to pin down the valve knocker locker?

Not entirely...

NBBs function by releasing the valve knocker almost immediately after striking the valve.

A GBB functions differently, in that it requires the slide to act as a timing device to trigger the disconnect lever. This much you have shown to have an understanding of, and by holding down the disconnect lever, you are effectively allowing the valve knocker to pull freely away once it's done its job.

The problem here, is that an NBB not only allows the valve knocker to freely pull away after the strike, but it also disengages the hammer mechanism. I'm not sure how modern Airsoft NBBs work, whether they actually hit a firing pin or not, but in the old gas NBBs I played with, the hammer was just for show. The trigger would pull/push the firing pin (valve knocker) against the flow valve, and when the pin was pushed past a certain point, a sear or some other type of mechanism will release it from the trigger, causing it to pull back from the fill valve and place it back in to it's natural resting position, which is away from the flow valve.

In a Hi-Capa, even if you were to disengage the disconnect lever, the hammer is STILL applying pressure to the valve knocker. As long as the hammer is down, it will cause the valve knocker to vent gas.

Have you ever manually decocked a Hi-Capa (with the decocking mod on it) while a gassed mag is inserted? That's right... the slide will blow back and strike your thumb, unless your main spring happens to be very weak and you allowed the hammer to ever so gently come to a rest against the valve knocker. The hammer won't be fully down, though, and if you pushed it, it would engage the valve.

Without the slide blowing back and pulling the hammer back for the next shot, the hammer will continue to dump gas, and you'll be back at the same spot.

A new disconnect lever needs to be created to disengage the hammer from the valve after the strike. You may have to use some type of internal mechanical device to achieve this.

[spazzytroy]

@illusion: Not sure I follow. If I insert a fresh mag into an uncocked pistol, the replica's hammer is down(not the valve knocker), and not dumping gas.

@madmax: I think there is some terminology confusion. Are you suggesting I tape down the locker to part that hits the valve to release the gas?

[MadMax]

Yes, tape down the bit that sticks out of the top of the frame on the left side.

[spazzytroy]

I tried essentially doing that by filling the the hole it extends into, like in the photo except all the way across. Maybe I'll try tape, just to make sure it gets held down and it's not the fault of my insert.

[ILLusion]

Originally Posted by spazzytroy

@illusion: Not sure I follow. If I insert a fresh mag into an uncocked pistol, the replica's hammer is down(not the valve knocker), and not dumping gas.

That's correct. This is because the valve knocker has a secondary feature designed in to it to allow it to swing UP in the event it is extended forward when a magazine is inserted. The extended flow valve in the magazine magazine will push the extension *UP* and out of the way.

Following that, the first time the slide is racked back, it will consequently lock the hammer back as well as trip the disconnect lever, which allows the valve knocker to be pulled backwards in to its natural resting state.

Once pulled back, the valve knocker is now clear of the extended flow valve and will drop down in to it's normal position and primed to strike directly forward against the flow valve. Once the trigger is pulled, the sear clears the hammer, which allows it to strike forward against the valve knocker which strikes directly forward in to the flow valve to create the shot.

You need to create a method that will internally release the connection between the hammer and the valve knocker if you want the gas flow to be disconnected. If you're not using the slide as the timing mechanism, you'll need to figure out another way to do it, which is why I think this would be anywhere near an easy project.

[spazzytroy]

Originally Posted by ILLusion

The extended flow valve in the magazine magazine will push the extension *UP* and out of the way.

I wonder if i could use the action of the trigger getting pulled to push the valve knocker up and out of the way, simulating a mag getting inserted. By doing this I'd bypass needing to screw with both the BBU and the knocker locker. Getting the timing down would be tricky though, and I can feel the crispness of my single action trigger slipping away while repeating mode is engaged heh

[ILLusion]

It's possible... you'd need to create some type of levering system that you can easily engage and disengage externally.

Single stage or double stage single action firing would pretty much disappear at that point. If you've ever fired an NBB or a self-disengaging system as described, you are looking at a double action.

[spazzytroy]

Originally Posted by ILLusion

It's possible... you'd need to create some type of levering system that you can easily engage and disengage externally.

Single stage or double stage single action firing would pretty much disappear at that point. If you've ever fired an NBB or a self-disengaging system as described, you are looking at a double action.

Actually I may have an idea to keep my single action as well as implement this mod.

A picture is worth 1000 words so...


With a simple plate, the thing in red (guess I'll call it a 'valve knocker blocker' HA!) I could adjust the trajectory of the valve knocker as it comes forward. This plate would be easy to engage/disengage w/ a simple external lever that I've already figured how to rig. I have the stuff to mill/cast a new valve knocker if this doesn't work out so I'm not too worried. What do you think?

[ILLusion]

Hard to say. Run a prototype.