GBBR fps reduction for TM/WE

[renegadecow]

I often come across people complaining how a certain gun shoots particularly hot and using a lower pressure gas isn't exactly a viable solution. While you could buy an NPAS where applicable, for TM/WE spec rifles (pistols too, but hardly applies as they shoot low enough on their own) there's a chance for it to adjust itself with use and there is the cheaper, less known option: making a restrictor disc.

 

Start by taking apart your bolt down to the nozzle. In this case, a WE G39.

 

Then you find yourself a piece of polycarbonate or anything really that can take the pressure and you can cut/drill easily. Some have used AEG shims but I choose to use polycarbonate as I get lots of scraps from making lens protectors like these for EOTechs.

 

Then cut out the discs. The outside diameter isn't too important, around 7.30mm or as long as it sits flat in the nozzle. The inside diameter is what is crucial as this is what impedes gas flow. The process itself is trial and error, but once you get the desired fps the process is essentially repeatable. In this case I make 3.00mm holes which brings my AKS74U from 430fps to 360fps on .20g. I could have made the hole bigger as my site limit is 420fps, but since I'm using .30g bbs exclusively, the Joules match up to the equivalent of  420fps on .20g. I haven't chronoed this gun yet, but given the longer tight barrel in it I'm guessing it will do right around 390fps.

 

To install the disc, you just place it in front of the floating valve spring.

 

And looks like this from the front.

[Stuey]

On fire today aren't you?

 

So nothing extra required to hold the disc in? Spring is enough? I guess the gas flow is in the right direction to help it stay in too, rather than the opposite.

[intinerious]

On fire today aren't you?

 

So nothing extra required to hold the disc in? Spring is enough? I guess the gas flow is in the right direction to help it stay in too, rather than the opposite.

 

There's always pressure being applied to the disc since the floating valve is always pushed against the stopper inside the nozzle, so the disc itself will always be pressed against the inside of the nozzle head by the FV spring.

[bankz5152]

Smart.

[renegadecow]

You can glue the disc on to be sure but from experience it doesn't need it. Once it's in there's no way for it to come out unless the nozzle breaks.

[bettyblue84]

Nice little guide there man.  May try this ou.

[loki7491]

Nice. Some cool engineering there dude.

 

I don't want to be "that guy" but another method (and a little easier) is to glue a small o ring to the rear of the flute valve. This forces it to sit forward further in the nozzle meaning the dwell time is less for the gas to get down the barrel.

 

Not my intent to jack your thread as I really like the idea. Just hoping to add to the pool of ideas.

[Atsalakotos]

Nice writeup! I have done the same for a couple of years now and never had any problems... Also it is a lot more stable than an NPAS will ever be..

[bentwings]

This is a neat idea. I got here from another forum by asking how to prevent the npas from rotating while attempting adjustment.

 

While it is possible to stick a small piece of welding wire in the gas port and adjust the screw with your other hand, there is not a lot of room in the mag well. Squeezing two big fingers down in there with the bolt held back by a marginal bolt catch is not easy.

 

Using the calibrated washer idea is reasonable however it does mean about 75 % disassembly of the WE M14 for each adjustment. I have 3 nozzles so I could make 3 different size orfaces and get a spread of sizes so I could select the one I need.

 

Thanks for the info

[renegadecow]

I've finally figured out why using the restrictor disc somehow reduces the recoil impulse on some guns. The disc acts as a spacer on the floating valve springspring which are pretty stiff on WE rifles to begin with making it harder to close, and in some cases it doesn't close completely which causes a leak for the the blow back cycle. The simplest fix for this is to reduce the springs length by about 2 dead coils and flattening the cut end.

[Feyd]

I've finally figured out why using the restrictor disc somehow reduces the recoil impulse on some guns. The disc acts as a spacer on the floating valve springspring which are pretty stiff on WE rifles to begin with making it harder to close, and in some cases it doesn't close completely which causes a leak for the the blow back cycle. The simplest fix for this is to reduce the springs length by about 2 dead coils and flattening the cut end.

How thick is the spacer on the affected guns? Is it something that using an AEG gear shim cut to the same port dimensions would solve?

[Stuey]

Using an AEG shim is one way of doing it, they drop into a lot of nozzles unmodified.

[renegadecow]

Yeah, that brings me to another development. Working on another M4 today it turns out the shim (1mm in thickness) is a tad too thick and keeps the floating valve on some guns from going forward completely leading to a leak. The spring alone isn't always responsible in keeping the floating valve from working properly, but consider also that cutting the spring in an effort to make it weaker also reduces fps a bit. Just don't cut it too much as that would lead to other problems. Basically, to make the polycarb restrictor disc to work you need to make sure the floating valve is able to close completely. If it can't, you'll need to remove material at the front of the floating valve. You will also need to check if the floating valve spring can fit too when fully compressed, at worst you cut two dead coils at the end like I did. With that, from a stock M4 length barrel on green gas and 27°C I was able to mess around with the following results:

no restrictor, untouched spring = 420fps

no restrictor, cut spring = 390fps

3.5mm restrictor + cut spring = 350fps

3.25mm restrictor + cut spring = 320 fps

3.0mm restrictor + cut spring = 300fps

 

These only serve as rough numbers. For example, I just put in a 3.5mm restrictor + cut spring on my AK and the result was 390fps. Taking the restrictor out puts it at 410fps and completely stock does around 440fps. I was able to run it properly with an uncut spring and a 3.0mm restrictor before which did about 360fps. People have used AEG shims too. They work the same way and likely will not need the modification of the floating valve or its spring being very thin, but will be harder to modify the size of the hole to adjust for fps.

[jkpics]

Just put the *albatross* end of a 7,5mm drill in one of the new MSK/M4 nozzles, calipers says 7.43mm so put a 7,5mm router bit in it and now you can just buy 7.5mm x 3mm x 0,2mm and open the hole up as needed, I think it will need a bit of force to get it in so it will not drop out or shift around.

[renegadecow]

It doesn't have to be tight fitting. The pressure from the floating valve spring keeps it in place.

[heroshark]

Good trick .I used an old valve with the shoulder taken off and the innards taken out ,I then cut a notch in either side on the end and screwed it into the end of the nozzle on my pdw .

[renegadecow]

Now that'd be really convenient. Threaded nozzles like jets on a carburetor.

[heroshark]

yeah it is, mainly did it as the stock nozzle is glued shut

[jkpics]

Sure, but the part I really like is not having to cut it my self in plastic.

[Feyd]

That's an interesting way of doing it, some of the clone L96s come with a similar restrictor, but it's way too big to fit inside a GBBR nozzle, might have to see if this fits a GHK...

 

*Thinks for a sec, hmmm, glued shut PDW nozzle... Was that the GHK?

[heroshark]

*Thinks for a sec, hmmm, glued shut PDW nozzle... Was that the GHK?

 aye