[Cowboy]

Hey guys.
I simply cant figure this one out.

I have a brand new king arms PDW (basicaly an m4 that takes mp5 mags) I tested fps with the stock spring (m110 from what im told) and it was shooting 330fps, wanting it for a field gun I proceeded to open up the gearbox and install:
13:1 gears
Gate Aster
oring shs nozzle
SHS full metal tooth piston
SHS piston head
M125 spring
reshim.

I expected to see 400+fps, as upon checking I am air sealed nicely...Not the case, its coming in at about 360fps

Is there something I am missing? The only thing I can think of is something with my gearbox-hopup. but even still, why was I getting 330fps on a m110 with less then ideal air seal, and now getting a small 30fps gain with better seal, and much larger spring?

Any pointers would be appreciated.
Thanks.

 

[danerd]

How long is the inner barrel, what type of cylinder are you using (if ported, where is the port located? is it not ported?) There is a phenomenon that occurs where you can combine a [very] short barrel with an over-volumed cylinder and lightweight ammo and the BBs will actually leave the barrel before the gearbox is finished cycling. Its enough to cause minimal FPS gain/ FPS loss in some setups despite having a heavy spring.

 

[Lefse]

Ah yes, the "MP5K syndrome", AEGs with very short barrels tend to have a low compression efficiency. Id also retry with the stock aurora nozzle to check for fitment issues. Adjusting the piston weight might also work.

This is in addition, to what Danerd mentioned before. Stock AEGs are actually often over volumed. Ive found that a 1/2 type cylinder has enough volume for at least a 300mm barrel with .25 gram bbs.

 

[Cowboy]

How long is the inner barrel, what type of cylinder are you using (if ported, where is the port located? is it not ported?) There is a phenomenon that occurs where you can combine a [very] short barrel with an over-volumed cylinder and lightweight ammo and the BBs will actually leave the barrel before the gearbox is finished cycling. Its enough to cause minimal FPS gain/ FPS loss in some setups despite having a heavy spring.

inner barrel is 180mm
cylander is ported, however is very far back (I would say 80% or more)

So the suggestion would be to throw one of my G&G cylinders in (I have one that is ported halfway down) or try using a longer barrel?

Thanks again!

 

[Cowboy]

Ah yes, the "MP5K syndrome", AEGs with very short barrels tend to have a low compression efficiency. Id also retry with the stock aurora nozzle to check for fitment issues. Adjusting the piston weight might also work.

This is in addition, to what Danerd mentioned before. Stock AEGs are actually often over volumed. Ive found that a 1/2 type cylinder has enough volume for at least a 300mm barrel with .25 gram bbs.

So, I just stole my 300mm barrel from another gun...the FPS is now 280fps lol

I did the "tissue paper" test on the hopup, and passed with flying colors, I am slightly baffled now.

 

[Deus_Vult5687]

You might want to try using all of the stock compression parts (including the barrel) again too see if the problem goes away. That would possibly rule out a big portion of causes, in my mind.

If the problem persists, the cause is most likely not due to the compression parts; the opposite is true if the problem goes away.

 

[Jimps]

Yea that sounds like normal FPS(330) for M110 and 180mm barrel. You have no issues.... If your FPS went to 280 with a 300mm barrel, it probably has to do with the hop/rubber/barrel/nozzle combination, not internal air seal parts.

 

[Cowboy]

You might want to try using all of the stock compression parts (including the barrel) again too see if the problem goes away. That would possibly rule out a big portion of causes, in my mind.

If the problem persists, the cause is most likely not due to the compression parts; the opposite is true if the problem goes away.

Sounds good, I will give that a try and see what happens.
Thanks

 

[rm105]

What brand spring did you buy? If SHS or related, not to uncommon for them to be incorrectly labeled.

Seem to remember King Arms gearboxes being picky with some aftermarket parts, might check your tappet timing in particular while the shell is open

 

[Lefse]

So, I just stole my 300mm barrel from another gun...the FPS is now 280fps lol

I did the "tissue paper" test on the hopup, and passed with flying colors, I am slightly baffled now.

Thats weird. Piston weight? Orings sealing well? Is the fps consistent? Tried different bb weights?

 

[Guges Mk3]

M125 springs are known to be "inconsistent" who is the maker?

With each stroke of the piston the AEG creates a pressure wave.

Going short barrel, large volume cylinder and light bbs, the pressure wave is never maximized...bb has left the barrel before the piston hit bottom...

Also...o-ring nozzles can cause cylinder tube drag...which causes other issues.

To get FPS up in a short barrel system...you need to up spring pressure and short stroke the system to compress the pressure wave.

 

[Deus_Vult5687]

Also...o-ring nozzles can cause cylinder tube drag...which causes other issues.

Could you elaborate further on this? I'm not the OP but I haven't heard of this before and am curious because I use o ring nozzles in some of my guns.

 

[danerd]

O-ring nozzles cause friction on the cylinder head tube. Friction causes drag and can lead to tappet timing issues. Non O-ring nozzles don't cause as much friction and therefore don't have as much drag. This can be remedied by using a quality lubricant and routine maintenance.

 

[Deus_Vult5687]

O-ring nozzles cause friction on the cylinder head tube. Friction causes drag and can lead to tappet timing issues. Non O-ring nozzles don't cause as much friction and therefore don't have as much drag. This can be remedied by using a quality lubricant and routine maintenance.

So I am assuming that you would notice that friction more in high rpm setups? I suppose that if things are cycling a lot faster, any drag, especially in the compression area, could cause pressure loss and maybe even feeding issues.

Anyways, I am now speculating because I am currently in the process of pushing the limits on an ssg build and am using a double o ring nozzle. I thought some grease on it would be enough to reduce most of the friction, but I might be wrong.

 

[mr. potatohead]

I'd for sure ditch a double o-ring if you're pushing 40rps SSG. Single o-ring might be viable, but I think something like a laylax/prommy pom nozzle would be a much better choice.

 

[Deus_Vult5687]

I'd for sure ditch a double o-ring if you're pushing 40rps SSG. Single o-ring might be viable, but I think something like a laylax/prommy pom nozzle would be a much better choice.

Thinking more like ~50 rps lol.

I'll keep your advice in mind. I don't know why I didn't think about it myself at the time, I guess I have just been so focused on getting the gearbox to cycle properly ha ha.

I'll stop hijacking OPs thread though; thanks for the info guys.

 

[Cowboy]

M125 springs are known to be "inconsistent" who is the maker?

With each stroke of the piston the AEG creates a pressure wave.

Going short barrel, large volume cylinder and light bbs, the pressure wave is never maximized...bb has left the barrel before the piston hit bottom...

Also...o-ring nozzles can cause cylinder tube drag...which causes other issues.

To get FPS up in a short barrel system...you need to up spring pressure and short stroke the system to compress the pressure wave.

Yes it is an SHS spring.

I will try chrono with .3 bbs and see what the fps is out of curiosity.
Ill also look at that nozzle (if I remember correctly it didnt have much more friction to it then the stock one though)

One question I still have is when putting a 300mm barrel in, why the drastic drop in fps? with my cylinder ported at 3/4, I should have plenty of volume for the longer barrel.

 

[Jimps]

If your inner 300mm barrel is exposed, take your hand and push it in towards the gun while you chrono to see if there is a change in FPS.

 

[Guges Mk3]

yeah SHS springs are not "consistent"...they have a large variance level...

Talked to a ME professor last weekend, who teaches at UW Stout on Fluid Mechanics.

After explaining the history of airsoft and then some parts, I went to o-ring nozzle...she said...not needed if the nozzle is made to be just slightly larger than the air tube.

She said you can do a float test on the nozzle and if the nozzle slow settles on the base of the cylinder head after you seal the tip...the compression is good enough that nothing will pass backwards during normal use.

Which is the spec level of Krytac and Tokyo Marui