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rootuser

Odd thing happening with KJW m700

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Hi all,

 

I am new to the KJW m700 and Tanaka style guns as a whole, but I am not new to Airsoft, AEGs or Gas powered guns like paintball or airsoft pistols etc so that's why I am a bit perplexed here.

 

First off I just built myself an HPA set-up. I have a 3000 PSI HPA tank (800 PSI output), a Palmers Stabilizer, Rap4 Coldshot (instead of HPA when desired), quick disconnects etc. I tapped a normal KJW short mag for testing.

 

The gun itself has a full length 6.03 Barrel, Firefly bucking, KA Barrel Nut, KA Bolt Spring, G&G stiker. The GG test mag has the King Arms plunger set.

 

When I use the GG magazine and I shoot .20s I chrono at 400 FPS pretty consistently. +/- 5 FPS.

 

When I chrono with the same GG magazine with .40s I get 460 FPS...........?????????????? huh?????? A bit less consistent (its +/- 10 FPS).

 

When I chrono with .43, I get around the same 460 FPS I get with the .40s.........

 

 

I have checked this on multiple chronos and checked the weight of the BBs. This IS happening. Can anyone tell me why??????????

 

What does 460 FPS with .40s = with .20s? I really don't want to injure some one because im shooting too hot and don't even know it.

 

 

Also, do the stock mags have any way to turn up or down FPS? I haven't messed with that yet, but im concerned if I can't get this stock magazine to lower it's FPS I'm going to run in to other problem eventually.

 

Also, seeing what I've done with this gun so far, can any suggest any hop-up parts that would help?

 

 

Thank you much for helping out a newbie to this type of weapon.

Edited by rootuser

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Heavier BBs cause the gasses to have more time to expand behind them. Never heard of the effect being this pronounced though.

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Certain brands will have ball size and roundness that are inconsistent from one to the other. Example: KSC .25's are 5.90mm while Airsoft Elites are 5.96. The tighter the barrel the more the effect will be prevalent. However, I've never seen anything with a gap that large. What brand are you using? You're testing with the hop up off right?

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Certain brands will have ball size and roundness that are inconsistent from one to the other. Example: KSC .25's are 5.90mm while Airsoft Elites are 5.96. The tighter the barrel the more the effect will be prevalent. However, I've never seen anything with a gap that large. What brand are you using? You're testing with the hop up off right?

 

 

Yes hop-up 100% off.

 

I am using Madbul .40 and .43s and Excel .20s. Its really rather perplexing. It seems to be with just this one magazine that the effects are this odd. Other magazines seem to be more predictable.

 

 

Looking at the magazine exploded drawing I think part 98, the Arm Plunger O-ring is leaking very very slightly. You can hear a bit of something if you lift the magazine to your ear and listen for a while. I filled up the mag and I will see if it leaks it all out by the morning but I doubt it. Also looking inside the magazine itself, it seems like the magazine valve is turned in such a way as to have just enough room for the knocker arm to fit and that's it. In another magazine of mine this is wide open.

 

Is this a way to turn up and down the FPS? Maybe I've got some adjustment all out of sync?

 

Thanks for the help.

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As it has been said, increases in output energy as you increase BB weight is not all that uncommon in gas rifles. I had done some testing awhile back ( http://www.airsoftforum.com/board/Power-fl...-m-t171613.html ) and while that data is not "accurate" to what I've seen recently through the same gun, it does show fairly large increases in energy as you increase weight. There's a spreadsheet down in post #22. So, while the numbers aren't quite "right" (in that my newer data shows much higher velocities, but basically similar proportions), it's still showing generally what I've seen. My most recent data puts my gun shooting about 600fps with a .20g (3.4J), about 545fps with a .28g (3.9J), and then 485fps with a .43g (4.7J) with propane. So, while it's not the 150-200% increase in energy that you're seeing (where I see about a 50% increase), increases in energy are normal.

 

Also, are the Excel .20s you're using the biodegradable type? If so, that could possibly explain the lower energy levels you're seeing. I've found Excel bio .20s to have an average diameter around 5.85mm (with BBs ranging from about 5.80mm to 5.89mm), where Madbull .43s were closer to 5.93mm. So, while that's not going to compensate for all of the difference, it will have some effect.

 

Also, to answer a couple of your questions, 460fps with a .40g is equivalent to a .20g at about 650fps. It's a bit higher than most fields probably allow. To decrease the velocity, you could use a weaker gas, like duster gas, though I'm not sure if I understand your situation, since you mention a GG (presumably green gas) mag, yet you mention having an air rig. If it is actually attached to the air rig, then you could try adjusting the output pressure from the regulator. I found in my AW338 (with "brief" testing one day) that duster gas drops the velocity of my gun considerably, and in addition, it gave me much more velocity consistency compared to propane and many more shots per fill. I dropped down to about 400fps on duster compared to about 550-600fps on propane using the same .20g BBs, and while I don't know how many shots I could have gotten out of that single fill, I did fire about 120 shots on a single fill of duster without any significant change in velocities (I.e. the first shot of each set being 410-420fps, with subsequent shots leveling off around 380-390fps) before I decided I was wasting ammo, at which point I emptied the mag of gas, and it appeared to still have a lot left. In comparison, on propane, I noticed changes in output velocity in other testing after as much as 30 shots (I had been doing testing, then after 30 shots, I decided to refill the mag, and then right after doing so, the output jumped a consistent 20%, though it is possible there was minor leaking). One of these days, though it is a waste of duster gas, I should see how many dry fire shots I can get from a single fill of duster gas, because I could easily see it getting into the 150-200+ shot range based on what I saw previously.

 

Another possible solution for decreasing the output power might be to get a shorter inner barrel. It would give the BB less time to build up speed with the gas behind it, or at least I assume it would. I don't know if it's possible to do that in a KJW with the outer barrel, since I'm not really familiar with that gun.

 

Anyway, while my gun isn't the same, it's the same general idea with the gas power and changing output velocities by changing weight. As spider_child said, I'm also surprised at that increase, since it is a rather large jump, and a .20g should still be heavy enough that it would have a higher velocity (even if it was only like 50fps) compared to the heavy BBs in a long barrel.

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Possible theory from basic physics:

 

The whole point of barrel length is to give the bb time to accelerate until it reaches its max kinetic energy.

 

Heavier bbs have greater mass, so they have an even bigger ability to "store" kinetic energy/inertia.

 

At some point, it'll take *more* energy to accelerate lighter than something heavier, each with equal surface area.

In martial arts, the heavier something/someone is, the easier it is to keep moving with just a little bit of energy. The trick is to get it started.

 

Sniper barrels are usually a lot longer than other ones, so they might just be long enough to push a .20 bb to its maximum kinetic energy potential in a KJW M700 from whatever gas propulsion you're using.

Meaning each gas has a maximum amount of energy it can give a bb, and each bb has a maximum amount of energy it can keep.

 

So there *might* be enough length inside the barrel for a .20 bb to reach its maximum, with some length left over. This makes the .20 bb harder to push once its maxed out, while a heavier bb that's now moving can still take on more energy.

 

So maybe what's going on inside your M700 is that if you use the same gas/psi each time, it has a max amount of energy it can give to a bb no matter its weight, and if the gas is strong enough to NOT max out on just a .20 bb, but has energy left over for a .3, then yeah, it'll go faster.

 

Maybe.

 

Good times,

-Sav.I

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Whoa whoa, hold up there guys.

 

He's talking raw FPS not joules/energy. His velocity ought to be decreasing when moving to a heavier BB. Nothing about energy in there. This is purely mechanical.

 

This is also the first time I've heard of this happening without tampering or changing the regulator. Even in cases of a physically larger bb's vs smaller bb's in an ultra tight bore the increase in fps wouldn't be this high when transitioning from .20's to double the weight with the .40's. If you were experiencing leaks or block back from the heavier bb's then you would be losing velocity. But Madbull's aren't much larger in diameter than KSC (believe they were 5.96's).

 

About the only thing I can think of is that while using the .20's you had a poor seal somewhere and when you reloaded and re-inserted the mag you got a better seal. This theory kind of fails if you've done repeated tests and the results are consistent.

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This phenomenon is called energy creep. Its not uncommon with gassers.

 

Theory behind energy creep as a gas guns only thing has been thrown out (it's weight related and effects spring and gun). Either way, that is energy based not velocity. Even with energy creep theory his fps should be dropping with heavier rounds, not increasing.

 

Next person who mentions energy in here as an explanation will have a hair sculpture made from my back hair and burned as an offering to the Evil Pikachu! :a-angry:

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Theory behind energy creep as a gas guns only thing has been thrown out (it's weight related and effects spring and gun). Either way, that is energy based not velocity. Even with energy creep theory his fps should be dropping with heavier rounds, not increasing.

Next person who mentions energy in here as an explanation will have a hair sculpture made from my back hair and burned as an offering to the Evil Pikachu! :a-angry:

 

WTF are you talking about? The phenomena is called energy creep. And is vital to the conversation.

Assuming the bbs are actually shooting hotter with heavier weights, this is just an extreme case.

 

And energy creep is more apparent in gas guns because of the way of expending gas. I never said the phenomena is exclusive.

I only skimmed the first posts, so this may have been covered. Since the bb is heavier, it builds up more air behind it before it starts to move. This creates a higher pressure zone behind the bb, which make it shoot it harder than a light bb. Get it?

 

My theory is as follows

This magazine must have something wrong with the faulty valve that will get the bb to hold the high pressure longer.

It shoots gas over a longer period of time that will create this extreme energy creep.( A crude way of explaining it would be to say that it spends more time adding gas to the bb, because the valve acts slower) That, in conjunction with an abnormally thick bucking would help to hold the bb in place, might just create this thing you are experiencing.

Edited by RunnerGunner

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Theory behind energy creep as a gas guns only thing has been thrown out (it's weight related and effects spring and gun). Either way, that is energy based not velocity. Even with energy creep theory his fps should be dropping with heavier rounds, not increasing.

 

Next person who mentions energy in here as an explanation will have a hair sculpture made from my back hair and burned as an offering to the Evil Pikachu! :a-angry:

 

 

Ok do I will do some testing and write back with findings. I have one quick question first:

 

What is the max PSI you use in air rigs to maximize FPS? For example at 850PSI the hammer doesn't even hit hard enough to activate the magazine (yes I have the upgraded KA spring).

 

 

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Ok do I will do some testing and write back with findings. I have one quick question first:

 

What is the max PSI you use in air rigs to maximize FPS? For example at 850PSI the hammer doesn't even hit hard enough to activate the magazine (yes I have the upgraded KA spring).

850psi into your magazine? you are going to blow yourself up, buddy.

 

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850psi into your magazine? you are going to blow yourself up, buddy.

 

 

Haha probably. But I'm doing this in a controlled environment and I am protected. I can't dial up more than 850 PSI atm because the bottle im using won't do more. I would guess the magazine itself can hold well over 1000 psi without a problem.

 

I was just wondering at what point can the plunger be depressed normally by the trigger/bolt?

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WTF are you talking about? The phenomena is called energy creep. And is vital to the conversation.

Assuming the bbs are actually shooting hotter with heavier weights, this is just an extreme case.

 

And energy creep is more apparent in gas guns because of the way of expending gas. I never said the phenomena is exclusive.

I only skimmed the first posts, so this may have been covered. Since the bb is heavier, it builds up more air behind it before it starts to move. This creates a higher pressure zone behind the bb, which make it shoot it harder than a light bb. Get it?

 

 

I understand the point you're trying to convey, unfortunately it's not really valid. First he's using an air rig. Neither Co2 nor HPA expands when it's released past the regulator. That happens within the container where the liquid "boils" into a gaseous state. The pressure is uniform when applied to the bb. If any pressure builds up the regulator responds by reducing the amount of gas released from the poppet valve which is what they were designed to do. Of course it doesn't work the other way if there is a leak as it's only job is to release gas at the pressure dialed in.

 

If this was a propane or green gas only gun then I can see the confusion. Since propane sits at a much lower psi, some of the gas used to propel the bb might actually have come from liquid that "boiled" while before the valve shut off. Unfortunately the problem there is that the pressure exerted by propane/green gas is very limited and does not build up. The gas conversion does not allow it to "build up" past the pressure needed to keep it at a liquid state.

 

Third, the bb's are of different weight but the same size. If the lightweight bb was much smaller than the barrel's internal diameter and suffering from inefficient use of air pressure while the heavier bb was larger and filled the gap making it more efficient then you we would have an explanation....up to a point. The problem being that even with that logic the heavier bb would be breaking even with the lighter bb in feet per second.

 

The fact is that with every rifle a bb that is almost twice as heavy as a .20 should have a SUBSTANTIAL decrease in velocity at the muzzle compared the the .20 bb. But 60fps above it is phenomenal.

 

We won't know more until we see some more test results.

 

850psi is a great way to bust your seals. And permanently deform the magazine. Depending on your setup the pressure should not go above 160psi (Tanaka owner with ultra-short barrel and coiled remote runs this psi) with an average operating range somewhere between 80-120psi. Results may vary.

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I understand the point you're trying to convey, unfortunately it's not really valid. First he's using an air rig. Neither Co2 nor HPA expands when it's released past the regulator. That happens within the container where the liquid "boils" into a gaseous state. The pressure is uniform when applied to the bb. If any pressure builds up the regulator responds by reducing the amount of gas released from the poppet valve which is what they were designed to do. Of course it doesn't work the other way if there is a leak as it's only job is to release gas at the pressure dialed in.

 

If this was a propane or green gas only gun then I can see the confusion. Since propane sits at a much lower psi, some of the gas used to propel the bb might actually have come from liquid that "boiled" while before the valve shut off. Unfortunately the problem there is that the pressure exerted by propane/green gas is very limited and does not build up. The gas conversion does not allow it to "build up" past the pressure needed to keep it at a liquid state.

 

Third, the bb's are of different weight but the same size. If the lightweight bb was much smaller than the barrel's internal diameter and suffering from inefficient use of air pressure while the heavier bb was larger and filled the gap making it more efficient then you we would have an explanation....up to a point. The problem being that even with that logic the heavier bb would be breaking even with the lighter bb in feet per second.

 

The fact is that with every rifle a bb that is almost twice as heavy as a .20 should have a SUBSTANTIAL decrease in velocity at the muzzle compared the the .20 bb. But 60fps above it is phenomenal.

 

We won't know more until we see some more test results.

 

850psi is a great way to bust your seals. And permanently deform the magazine. Depending on your setup the pressure should not go above 160psi (Tanaka owner with ultra-short barrel and coiled remote runs this psi) with an average operating range somewhere between 80-120psi. Results may vary.

 

 

A few points here. After doing very careful measurement, the magazine does not deform up to 850 PSI. This is for the short magazine. I will test the large magazine tonight and I fully expect the magazine will begin to deform at 400PSI or so due to the larger surface area of the side walls. As for the seals, yes, they begin to leak at ~500 PSI. At 700 PSI they leak significantly. This may in fact temper the deformation factor because there is in fact an escape for the air. That being said, I am using different magazines for all these tests.

 

Moving on from there, I will begin testing ~100 PSI and see what happens for the air rig.

 

I am going to test a different brand of green gas for the problem I brought up originally. I can't imagine that the brand of gas makes a difference, but, you never know although I really can't figure out what the difference would be other than volume of lubricant in the gas??? I'm grasping here.

 

I will report back in a little while. Thanks for all the suggestions so far. It's nice to have discussion with people who are willing to take chances on theories and let the group discuss so we can really figure out what the heck is happening.

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The fact is that with every rifle a bb that is almost twice as heavy as a .20 should have a SUBSTANTIAL decrease in velocity at the muzzle compared the the .20 bb. But 60fps above it is phenomenal.

I had actually made up a spreadsheet of energy levels, weights, and velocities the other day to see what sort of energy creep levels you'd need to be seeing in order to see an increase in velocity as you increased weight, but basically tossed it out after realizing it basically showed energy creep was most likely not the reason for this particular jump. While I forget the exact numbers, it was something that was seemed absurdly high to me the amount of energy increase you'd need to see to get higher velocities as you increase weight. From my testing with energy creep, I saw increases that basically show about a 40-50% increase in energy as you double the weight from .20g up to about .40g, which comes to something like 5% increase per .03g increase in weight. The numbers I was getting with the spreadsheet were more like an increase of .3J or 15% per increase in weight of about .03g. And, even in that situation, the increase in velocity from the .20g to the .43g was only around 10 or so fps. The numbers here might be off, but it's well beyond what I saw in practice so I didn't bother to post it.

 

Another idea I had come up with relates to the way you find massive drops in velocity in AEGs by having too long a barrel and that maybe somehow there wasn't enough gas being release from the mag when using the .20s, but thinking it through, I came to the conclusion that this idea would probably give the opposite results as what the OP saw. If the mag wasn't releasing enough gas, or not releasing for long enough, the heavier BBs, being in the barrel longer, would probably have velocity drops that would cancel out or even overcome the amount you'd normally see due to energy creep.

 

With the BB sizing, I agree that it wouldn't be enough to see this sort of increase. I saw drops of like 20-30fps per increase in BB weight in smaller increments, which makes the jump from .20g up to .40g more like 3 or 4 or 5 of these drops, which would mean we should be seeing closer to 90-150fps drops, not a 60fps increase. I see minor fluctuations with higher velocities with heavier BBs from time to time, but it's only in certain situations with closely weighted BBs and only minimal differences in the velocity. Energy creep and BB size might be playing some roles, but they're definitely not the reason.

 

It's a weird situation and I'd like to know what's causing it.

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Some quick notes on tests:

 

Green Gas Brand 1 on .40 MadBull (Avg over 20 shots, two different chronos):

Max FPS: 463

Min FPS: 438

Avg FPS: 451

 

Green Gas Brand 2 on .40 MadBull (Avg over 20 shots, two different chronos):

Max FPS: 489

Min FPS: 440

Avg FPS: 454

 

Green Gas Brand 1 on .20 Excel Bio (Avg over 20 shots, two differnt chronos):

Max FPS: 411

Min FPS: 398

Avg FPS: 402

 

Green Gas Brand 2 on .20 Excel Bio (Avg over 20 shots, two differnt chronos):

Max FPS: 408

Min FPS: 401

Avg FPS: 404

 

 

So bottom line, it seems the brand of green gas doesn't matter. For the most part the FPS difference between the green gas brands were within the statisical margin of error which I estimate is +/- 7% or so from what I've seen on other small tests like this (sorry, its been a long time since statistics in college, so my math could be fuzzy).

 

Also, with HPA ~120 PSI (hard to be exact on a 1500 PSI guage): .20 Excel Bio

Max FPS: 551 FPS

Min FPS: 504 FPS

Avg FPS: 522 FPS

 

The shots for HPA were very consistent even tho the min and max were further apart then I'd like, almost all were right around 520 FPS minus 3 of them.

 

 

 

So I still don't get how on green gas it is still shooting lower on .20s. Any ideas? I can now reproduce the problem with two different magazines. One new and one used quite a bit.

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I had actually made up a spreadsheet of energy levels, weights, and velocities the other day to see what sort of energy creep levels you'd need to be seeing in order to see an increase in velocity as you increased weight,

 

Here you go.

http://www.bbbastard.com/ATP/index.htm

 

also a different downloadable spreadsheet of energy used at our games. Our limits are based on energy hence the need to understand the subject.

http://www.texasairsoft.org/index.php?page=chrono

 

Substantial increases in energy but substantial decreases in average FPS as expected.

 

It's a weird situation and I'd like to know what's causing it.

 

You and me both.

 

If these reading are both from a single magazine then I'm stumped. I thought at first maybe it was a difference between two different magazines that might have physical differences changing this. Well that's out the door.

 

Hmm, green gas now eh? Did you by chance top off the magazine with more gas when you loaded up the .20's?

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The ATP doesn't help with what I was doing. I wanted a spreadsheet with small enough increments so that I could arbitrarily pick say 2J with a .20g, find out what its velocity is, then add varying amounts of energy to that 2J (seeing what sorts of velocity changes you see with adding increments of .1J, .2J, .25J, .3J, etc relative to different rates of changing mass) and then find out what the velocity with a different weight would be with that new energy, then do that many times. The ATP doesn't really help me with that. It primarily shows a single mass with varying energies or a single energy with varying masses. I wanted to see the various energies and velocities in relatively small increments.

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Hmm, green gas now eh? Did you by chance top off the magazine with more gas when you loaded up the .20's?

 

I did the tests again just to be sure (topping off after the attempts, PLUS going with .20s first instead of .40s).

 

Same results. :(

 

Two magazines, same results. :(

 

So, now I am looking in to hop-up parts I have put in. I am so baffled at this. I tried 3-4 other gas guns I own (pistols and m4s) and they all behave predictably. Heavier BB slower velocity.

 

Is there any ideas as to what I should look at in terms of parts that could cause this?

 

So far I have upgraded the plunger, knock arm, striker arm and spring.

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There could be something with the .20s that you're using. Is it possible for you to get another brand of .20s to do more testing?

 

 

Yes I am getting another brand this evening and seeing if anything changes. I have used three different chronos so I've eliminated that as a possibility. There are only a few things possible at this point.

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Yes I am getting another brand this evening and seeing if anything changes. I have used three different chronos so I've eliminated that as a possibility. There are only a few things possible at this point.

 

 

Tried a different brand of BBs. At this point I am thinking I am just going to accept what happens as I'm running out of things to check.

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What barrel are you using? Is it possible that there is a static charge developing between one set of bbs and some part of the gun and not the other? Perhaps some property of one bb causes more drag than the other. What are you're climate conditions? Is it possible there is more drag on one bb due to hummidity?

 

Do your bb's roll freely down the barrel? What do you're shots act like with hop(one bb requiring more than the other)?

 

I know it's a lot and some is really off the wall but hope I can spark some inspiration.

 

P.S. Is anyone looking for a PHD project?

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