AOSC Forum Archive

Finally got a hold of a chrono to test my DMR build with the KWA SR12. Here's the important build parts you need to know to help me:

Guarder full cylinder with gasket seal mod and about a quarter inch of sorbo to correct AoE.
SHS full metal tooth piston/blue, with 2nd tooth removed and 3rd filed down for AoE.
Mini-Teflon tape mod on stock bucking with r-hop.
Guarder SP150 spring.
Everything else affecting compression is stock.

I chrono'd 404 fps with 0.25g bb's in this SR12 build. This did not seem right, so I began trouble shooting.

I swapped the spring with a Prometheus 120 spring, which gave 375 fps with 0.25g bb's, only 30-35 fps less than when I used this spring in the originally stock SR12 setup (405 fps originally).

I then tested the SP150 spring in a KM4 Commando, with the cylinder changed to the SR12's stock type 0.5 cylinder, to see if the sorbo and cylinder combo was the cause. All other gearbox parts are stock in the Commando. With the SR12 upper assembly on it, it chrono'd 445 fps with 0.25g bb. With the Commando upper, it chrono'd 360 fps with 0.25g bb.

I believe I know part of the cause of the lower than standard velocity, but want to hear your thoughts on why I'm not getting near the velocity a SP150 should be giving. My testing so far lets me conclude that the sorbo/cylinder combo has lowered my velocity roughly 30-40 fps from it's previous build with the Prometheus 120 spring, and should be similar with the SP150 build. Though this does not account for this large of a velocity disparity from what it should be.

UPDATE: I cut up a pink rubber eraser and used it to replace the external hop up unit spring, in order to apply more pressure and create a firmer seal between the hop up and gearbox/nozzle. In the setup mentioned above of the SR12 upper on the Commando lower, my velocity increased to 484 fps with 0.25g bb's. That is a 39 fps jump from earlier. I encounter a problem with using this air seal mod when using the both the SR12 upper and lower together though. The gun will not feed. The bb loads in front of the chamber, but the nozzle does not push it forward into the barrel. But when the chamber is clear of all bb's, you can see the nozzle cycle as it should when shooting. I also tried using the Commando's barrel/upper on the SR12 lower, same hop up setup as in the SR12 barrel assembly, and found it also failed to shoot. The suspect of this feeding problem must be somewhere in the SR12 lower receiver.

UPDATE: The cause of the feeding issue is that the hop up is being pressed forward enough that part of it protrudes into the feeding hole. Investigating a fix.

captainkirk05:
Without seeing exactly what you did with some if your modifications, I am going solely off of your description.


- Sorbo Pad:
1. What is your specific Sorbo Pad arrangement?
One piece Sorbo, stacked Sorbo, and are there any Spacers on the Piston?
Typically, a 1/8 Sorbo Pad stacked on the stock Sorbo Pad, or vice versa, does the job as far as setting AOE position.

Picture: 1/8 Sorbo Pad stacked on stock Sorbo Pad:


2. Did you bore out the center hole of the Sorbo Pad.
I tell people this all the time and it makes a huge difference both in performance and in function of the Sorbo Pad.
Standard - Left / Bored out - Right


This is what can happen to the center hole when you do not bore it out.
This is because the Sorbo will settle in a bit as it breaks in, so it flattens out.
As the Sorbp Pad wears down and out, it can really flatten out.
The only place for material to go is inward at the center hole, especially if you both perma-sealed it in and used the correct size added Sorbo Pad, which should be larger than the stock Sorbo Pad and just a bit smaller than the Cylinder so you can actually fit it in.
Picture:


3. Is the Cylinder and Cylinder Nozzle clean and free of all sealant you may have used?.
I use Acetone to clean, then a Dremel with a steel wool polishing wheel.
This makes it super clean and gives a mirror finish.
Pictures:




Piston:
The SHS 15 Tooth Piston Body is (or was at one time) a bit longer than a standard size Piston.
At least, I had one that was indeed like this a while back.
I do have a new SHS 15 Tooth Piston in my parts bin now, but it seems to be the same body length as a standard Piston.

Pay attention to this and make sure your Piston is lined up correctly and also not adversely affecting your AOE.
Original SHS 15 Tooth Piston I purchased over a year ago beside a stock JG Orange Piston.



Main Spring: Guarger SP150.
This Spring can typically produce 575fps w/ .20's and Ported Cylinders when brand new.
Keep in mind a few points regarding Full Cylinders and Perfect Compression as well as Air Seal and FPS.
Full Cylinder:
The added air volume in a Full / Type-0 Cylinder will slow the return time down of the Piston/Piston Head and lower FPS by a typical 10-20fps.
Ported Cylinders allow for faster return times of the Piston/Piston Head and often result in higher FPS.
The difference is you will actually shoot farther with the Full Cylinder and the lower FPS vs the Ported Cylinder and higher FPS.
This is another example of where FPS has nothing to do with performance
Compression:

Perfect Compression / Air Seal:
If your Compression is perfect and your Air Seal is as well, then the shooting performance will result in incredible consistency and you will have no air leaks.
This combination also acts as an air brake slowing down the return time of the Piston / Piston Head lowering the FPS. Combine this with a Full/Type-0 Cylinder, and you then have even lower FPS.
Once again, as with the Full Cylinder described above, it is another example of an increase in performance yet with a lower FPS.
Also once again, proving FPS to be just a number, and not an actual reflection of performance.
You should expect 550fps w/.20's and more with your set-up

Also:
With your specific m150 spring, it could be a quality control issue or a mislabeled spring.
You don't seem to get the right fps no matter what gun or gearbox set-up you are using.
I used that specific Guarder m150 Spring in my KWA DMR for a testing cycle of around 50,000 rounds and its performance was great. I think you just got a bad Spring.


Barrel Lock Down / Pencil Eraser Method:


I developed this method for Barrel Lock Down using Split Hop-up Designs specifically.
Link: http://ncairsoft.org/forum/showthread.p ... op-up-Unit
It works great and there are some things to know:
- Forward Hop-up Unit Spring Post needs to be completely removed.
- Eraser needs to be cut slightly longer than the available slot space.
- The resulting performance of this mod is mostly noticeable when shooting at distances past 300'.

I highly doubt any feeding issues you are having are a result of the stock Hop-up Unit or its Split Hop-up design.
It may be possible your Forward Spring is worn out and needs to be replaced.
KWA Forward Springs are shorter and not as strong as TM spec.
You may find replacing with a TM spec Forward Spring solves the problem, or sourcing a spring from something else such as a Pen or a cigarette lighter and cutting to appropriate size will do the job.

Split Hop-up Unit designs get a bad rap for they are engineered to assist a Free Floating moving Inner Barrel.
I have tested this specifically and have found that these types of Hop-up Units can produce outstanding shooting performance to distances well past 300'.
The KWA Hop-up Unit specifically is a fantastic performer and the best engineered of all the Split Hop-up Unit designs in my view.


General Advice:
Feeding Issues:
1.Reset/re-install the BB-Ramp.
Though the BB-Ramp may look seated correctly visually, it is often the cause of feeding issues.
I highly suggest doing this.
2. Re-seat/ re-install the Bucking.
The Bucking may look seated correctly visually, but may have a wrinkle or a fold in it as it sits in the Hop-up Unit tube. A simple re-seating or complete re-install of the Bucking has solved many feeding issues for many Techs.

2. Troubleshooting:
Be careful to continuously open, close and re-open the gearbox to troubleshoot a problem, especially in dealing with feeding issues.
Many times, the source of an immediate issue can be solved without opening the Gearbox.
This will help avoid everything from stripping out Gearbox Shell screw ports, not losing shims as they sometimes like to fall off, and avoiding re-installing heavy Main Springs.

3. Be careful trying so many new things at one time.
No matter how qualified, experienced, or capable one might be as a Tech, bad things often happen when trying too many new things at one time.
Too often, it causes more problems than it solves.
It also does not have to be the greatest gun build ever or have the latest and greatest upgrade or mod to simply shoot great and better than it did before.

4. The right parts, and the right modifications, installed the right way, gets the right results. Every Time!
So be patient, focus on one thing at a time, do each step correctly, and the results will take care of themselves.

Thank you for the thorough reply Vanevery.

Sorbo Pad -

I used the stock sorbo pad on the cylinder head, in addition to a 3/16" sorbo pad from Clandenstine, and one of the thin neoprene sorbo pad covers. It appears to be a substantial amount of sorbo, and looked to be the best amount to work with correcting AoE.
As for the size of the sorbo I added, it was actually smaller than the stock KWA pad, possibly around 1/8" smaller in diameter. In other words the pad doesn't touch the cylinder walls. I did bore out the center hole to match that of the KWA stock pad.

Piston -
I stood the SHS piston next to the stock piston. They are the same size from what the eye can see. I will say that the SHS piston feels slightly lighter than the stock one, though I doubt that has anything to do with my problems. The AoE is at about 12 o'clock.

Sealant -
The cylinder is smooth and shiny where it needs it. The gaps around the edge of the smaller diameter sorbo has a miniscule amount of residue, but it is so close and confined to near the base of the cylinder head that the sorbo would have to be removed for it to interfere with the piston head. The cylinder nozzle is spotless.

Spring -
I'm not going to rule out that the spring could be mislabeled. Even if it was, it is incredibly stiff compared to even my 120 spring, so I would assume it is stronger. To put it this way, I can install the 120 spring without a sweat. The 150 causes my hand and arm to shake around, and I grunt a good bit when putting it in.

BB Ramp -
I switched bb ramps between the SR12 and Commando. Seated correctly. No changes from it.

Bucking -
I have been playing around with the bucking continuously. Trying different forms of Teflon modding it, removing and reseating it, etc. Nothing changes.

Questions -
-Does it matter that my Sorbo pad has a smaller outer diameter?

-Could part of my problem be the bucking? Even if it is perfectly installed and air sealed, could it still leak in the setup?

-I see how you applied the block eraser mod to the KWA hop up. You say the hop up is designed to be a free float, and it works as such to facilitate proper feeding. In my attempt to use the eraser, it removed the characteristics of the free float system that allowed it to feed. The amount of pressure the eraser puts on my gun makes it slightly difficult to close the receivers together too. Did I simply use too much eraser, and I need to shorten it a small amount to reduce the pressure it exerts when the receivers are closed, so as to allow movement of the hop up? If yes is the answer to that, then what advantage is there over simply using the spring, as the pressure applied would be close to the same at that point?

I think my stock hop up spring is at normal strength as well. I had one go bad a few weeks ago, and used my current one to replace it and bring my gun back to normal operating specs. Nothing had changed with it as of yesterday, so I believe it is still good.

And thanks for the info on how the cylinders can affect velocity and performance. I find such things to be trickiest to comprehend in airsoft.

vanevery wrote: With your specific m150 spring, it could be a quality control issue or a mislabeled spring.
You don't seem to get the right fps no matter what gun or gearbox set-up you are using.
I used that specific Guarder m150 Spring in my KWA DMR for a testing cycle of around 50,000 rounds and its performance was great. I think you just got a bad Spring.

I have tested multiple set ups now. Changed sorbo thickness down to 1/8", which yielded a 40 fps boost. The eraser mod adds another 30 fps, topping me out 70 fps higher than before, but I'm still working on the feeding issue. I will try shimming the hop up away from the gearbox with tape to allow feeding in combination with the eraser mod. All this said, my max velocity is 510 fps with a 0.20 on the sp150 spring. From my testing, I can theorize that my setup would be shooting about 550 fps +/-10 with a 0.20 if I were to remove the remaining 1/8" of sorbo, but that's not an option. It looks like I just need to get a stronger spring. I can't complain too much about the sp150's performance though, with all of my mods it is shooting over the advertised fps of 490 fps with a 0.20g. If other springs don't quite get me where I need to be, then there's always the option of a tight bore barrel.

I have some news. First the bad.

It seems the laws of physics applying to my gun have changed overnight. When initially trying the eraser mod, I was getting peak velocities of 514 fps w/.20. It wasn't sustainable though due to feeding issues it created. I tried shimming the hop up away from the gearbox to facilitate feeding in combination with a high pressure force eraser mod. This combo actually hurt my velocity, so after various combinations I threw that idea in the trash. I also damaged my last remaining stock hop up spring. NOOOOOO!!!!
P.S. The Lonex 70 bucking (unmodified) didn't work nearly as well as the stock KWA bucking r-hoped. Air seal problems dropped fps by 10 to 15.

And time for the good news:

As a spur of the moment "whatever I don't care" thought after ruining my last spring, I tested the gun with no hop up spring or modification. My fps jumped 10 higher, from the 470 - 480 range with all the combos, up to 490. YEP. I've been doing it wrong the entire time. Through various setups with different length erasers, I found that less pressure yielded better results, up to the point of having no eraser. I will keep an eraser that applies a barely noticeable amount of pressure, to keep my shot to shot consistency in the +/-0.5 fps I've been getting. With no pressure at all, velocity variation goes to about +/-2 fps.

So here is what seems so weird to me. At one game, a damaged hop up spring caused a massive air leak that made my 400 fps w/.25 setup with this gun drop all the way down to 230 fps. But now no spring at all makes it work better, and at higher air pressure?
Also, I have not been able to replicate the results of the 500+ fps chrono readings reliably. In one install an eraser mod with X pressure gave me such readings, and one takedown and reinstall later, it doesn't, or is followed by other issues such as misfeeding after a few shots. Weird huh... Quantum level stuff, where's Albert when I need him.

And in other news, what a surprise to see a spring give me the EXACT fps rating it's advertised at. Guarder sp150 is giving me 490 fps w/.20 on the spot. So this means choosing my next spring will be easier. I'm used to shooting about 45 fps over spring ratings due to KWA's design, but since the cylinder swap and sorbo mod my gun is pretty normal now.

And this, ladies and gentlemen, is why two piece hop ups suck.

But I'm glad you've figured it out.

To help with an explanation, it's possible the rubber buffer was pushing the hop up in a direction that wasn't straight back, possibly tilting it down ever so slightly. This would cause extra friction on the nozzle which could lead to air leaks.

I didn't see it in the first few posts, and kinda skimmed the last few... but did you test the compression of your air nozzle? Does it seal with the cylinder head when the air nozzle is extended?

Yay Star man is here for double help! I have figured "some" of it out, but I'm not out of the woods yet.

Yes I have tested out all the easy to test compression thingies.
I've done the typical cylinder compression tests, air nozzle included, and gotten great results. The air nozzle very, very slowly lets some air out, but that's typical of non o-ringed nozzles. I can feel the difference in air pressure coming out the air nozzle when shooting the gearbox. Definitely more pop than the ms120 spring I previously had, but that power is not translating out the end of the barrel.

As for the seal between the air nozzle and hop up, I have done both the flash light test, and the paper over the feed ramp test. Both tests are showing 100% seal. I have been playing with the thought of not having the nozzle kiss the bucking lip at the right depth or angle, but wouldn't these two tests reveal if that is the case or not?

If the nozzle / bucking kiss is the problem, possibly due to a change in tolerances from the batch my sr12 came from, then I may try to go about sanding down the front of the tappet plate. I don't know if this seems like an issue, but when looking into the hop up through the feed ramp, the nozzle barely touches the bucking lips. At most it seems it inserts itself a 1/2 mm or less.

Hmmm, shaving the tappet plate would definitely help push the nozzle further into the bucking lips, potentially increasing the quality of the seal. There's definitely no reason not to do the modification.

Is it possible that going a little too far can deform the bucking lips and cause a worse air seal?

That should only happen if something is misshapen/deformed.

I completed the tappet plate mod. I had to shave a little off where the plate rests on the sector gear axle too, simply taking it off the front near the front of the gearbox isn't enough to let it move forward, for everyone's general information. And sadly, nothing changed after doing this mod, other than my ability to go full auto with a high rate of fire build in the future.

My conclusion to this mod, is that if the tappet plate is already pushing the air nozzle into the bucking for perfect air seal, whether that air seal applies only at low pressure or not, this means the air nozzle is resting as far as it can rest in the bucking. The only way to change the nozzle's resting position in the bucking, given the tappet plate has the room to move further foward, is to use a stronger tappet plate spring to pull it further inside the bucking lips. Unfortunately, I have never seen a "Power Up Tappet Plate Spring" anywhere. So this is not an option, and honestly at this point, I don't think that would be the cure to my DMR air leak anyways, as no one else has to go to such extremes.

So next up on the list: trying out a new KWA hop up. Fingers crossed.

There aren't stronger tappet springs, what people usually do is cut the spring that's already there. Usually I cut one of the hooking loops off and bend a few more coils out to replace them.

I purchased a new KWA hopup and bucking for further troubleshooting, to see if either was the cause for an air leak. Once installed and tested, results were the same. Therefore to my knowledge, there are 2, and only 2 possible culprits left. One could be that the inner diameter of my particular stock inner barrel is a wider bore, allowing more air to bypass the bb under extreme pressures. The other possibility is that Guarder has made all of its springs weaker since earlier generations to have it's advertised fps accurate under perfect air seal conditions instead of the typical mediocre air seal conditions in most AEG's, but I haven't tested them in other guns so this is a shot in the dark.