Weird FPS

[captainkirk05]

I am working on a KWA KM4 Commando at the moment. Everything is stock, except for the Modify s110 spring in it. Chrono'ing it with this spring yielded some confusing results. In all my past experience, turning up the hop up decreases fps. Well, with this gun and the hop up off, I was getting anywhere from 300 to 312 fps with 0.25 gram bb's. Turning the hop up on to the normal playing amount bumped my fps to 320 - 325 fps with 0.25 gram bb's. Once you turn it up too far it begins to lower the fps as you would expect.
The only thing that has come to mind as the cause is some problem with the hop up rubber. Perhaps it is only getting a good air seal when the nub is pressing down on the bucking? Anyone have a similar experience?

[Star_folder]

Actually, what is happening is when you start applying more hop, you are slowing the bb down ever so slightly. That's what usually causes an fps decrease. But, sometimes it results in an increase in fps, because it creates a higher pressure behind the bb. Eventually, as you put more hop on, the friction from the hop overcomes the pressure spike from behind the bb.

This is generally a result of very good compression but a low fps, as you have seen. A higher fps puts more energy into the bb, breaking the pressure spike that would otherwise form.

[captainkirk05]

Interesting take on it. I guess something like an r-hop would help eliminate the gun switching between pressure spike on/off modes. I was going to put one in anyways since the 2G bucking is less than impressive.
Or for another fix, do you think changing the cylinder type would have any effect on this?

[Star_folder]

R-hop might help, but it might not. R-hop has a longer contact, so there isn't as much friction slowing the bb down, so it's possible you would see an increase in fps, but not to the degree that you are seeing now.

A cylinder change probably wouldn't effect it. It's the start of the air compression, the start of the bb path, the change in air is unlikely to have an effect at the beginning of the bb's flight.

[captainkirk05]

Update on gun funkiness. I installed the Guarder sp85 into the Commando. FPS results were 310 fps with 0.20g, and very consistent. Not as much of the weird fps stuff going on this time. FPS dropped by only about 5 when turning off the hop up.

To play around with the idea of hop ups causing the FPS weirdness, I took my KWA sr12's r-hop setup (sr12 hop up, bucking, brass ring, patch, nub, everything but the barrel) and put it in the Commando. Chrono results were much different. Max FPS was 292 with 0.20g. My variance was all over the place, from 270 - 290 FPS. I also did not have the odd reduction of FPS when the hop up is off. I am thinking possibly that parts from different production runs are not meshing as well (commando air nozzle and sr12 hop up), or maybe the bucking needs a little silicon oil on the lip to seal better. In the sr12 with this hop up setup my fps variance is much smaller, +-3 FPS.

On another note, the 2g bucking (stock hop up configuration) just sucks. I could not get it to lift a 0.25g bb tonight. I went through 100 rounds to see if it would "warm up". It never happened. It's definitely getting modified.

[Star_folder]

It's a common thing, for KWA buckings to be replaced because they offer no good hop up for heavier bbs. They work decently for light bbs, but, well, light bbs aren't very accurate anyway, so it really doesn't help anything.

[captainkirk05]

Can anyone pitch in and help me figure out ways to better the air seal of my hop up? When the air seal does it's thing right and seals good, I'm getting 298- 300 fps w/.025's, perfect for a CQB setup (installed a Guarder sp90 spring). It's just that quite frequently it loses this seal and drops down to 245 fps and everything in between. It will hit the 240's for a while, then decide it wants to hang out in the 260's for a bit, then 270's, then 250's, and every now and then it seats good and hits the 290's again. What tricks and tech'ing can I do to get a consistently good air seal?

[TokenTech89]

depending on where the seal is adjusting at, i would do the usual things: Strectch the o ring, teflone the cylinder head,polish and apply silicone grease in the cylinder to allow smooth cycling, teflon the outer part of the hopup to the barrel.

[Star_folder]

An o-ring air nozzle is also important, as that will leak just as much as anything else. It could also be your hop up. Try putting some rubber o-rings in front of the hop up chamber on the barrel. It's possible the spring isn't applying enough backwards pressure so that the nozzle isn't sealing with the bucking very well.

[captainkirk05]

I wrapped a layer of Teflon tape around the bucking, from the top all the way to the bottom where the brass barrel becomes visible, making a tight seal at that point, and also making the bucking a tight squeeze to get it into the hop up chamber. My best fps readings before using the Teflon tape: 300 fps with a .25. Now, with the funky bucking / hop up combo that was giving me the all over the place readings, my fps is ranging from 325 - 327 fps with a 0.25. EXCUSE ME! 325 - 327 FPS! That is a huge fps increase, and consistency improvement as well. You know what's horrible about that? This is supposed to be a cqb gun, it has a Guarder sp90 in it! I have an sp85 lying around, but it might not be weak enough!

[Star_folder]

Er, did you mean 325-327fps with a .25?

As for spring inconsistencies. Airsoft systems are different, super different, between porting, barrel length, and seal quality, you can have springs chroning all over the place. One spring rated with a certain barrel length and a certain ported cylinder with who knows what quality of air seal or the bbs they used, that spring will chrono completely differently than in an AEG that is completely different. It's complicated, and springs are rough guesses at best.

Personally, I go for a slightly heavier spring, and then short stroke a tooth or two to drop to the desired fps.

But! I'm glad to hear you've got these seal problems figured out, lol, sorry the gun is chronoing hotter than you expected though.

[captainkirk05]

Oh wow major dyslexia attack, I just fixed it. Yes 325 - 327 fps with a .25.

Having a perfect airseal is a PITA I may have to short stroke it like you say, or cut the spring, but I guess short stroking is easier. Yay more work!

[Star_folder]

Just remember you can't undo short stroking, can't really undo a spring either, but generally, a spring isn't as expensive as a set of gears.

[Dominum]

^ I always trim the spring (or use a different one) rather than short stroke for this very reason.