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Old 11-06-2019, 11:08 AM   #1
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Stuck on DMR build, velocity drop

Hey all,

So I got a round to building a DMR from the gun my sweet amazing wife bought me last year. I am stuck and frustrated and can't seem to solve this problem. I did some mods and I am getting perfect cylinder compression and very good compression at nozzle (not perfect because I am using stock nozzle with no o-ring). System is volume balance for .32g bbs.
Here is my issue.
Velocity w/.20 410fps no hop 390fps with nice slight hop
Velocity w/.30 330fps no hop 270fps with nice slight hop

Using a ML macaron bucking with ML omega nub. This is got to be the issue correct? I taken it out quite a few times now and cleaned it readjusted it and still similar results.

I am going to put the old bucking back in and try tonight.

Got a event saturday and really want to take my gun from momma. I do got the mb03 if all else fails.

edit:
Okay, so I flat hopped it with g&g green with same omega nub and getting much better results.
Velocity w/.30 330 no hop 320 with nice slight hop.
So I think either the macaron just wasn't gripping the bb enough (doubting this as it is pretty tacky) or more than likely causing a pinch jam of sorts in stead of gradually contacting the bb. This does stink because I have heard a lot of good about the macron and was super excited about it.

Still a little disappointed with slight velocity drop but I think that might have to do with my stock non-oring nozzle. Any thoughts?

Edit2:
Still wasn't happy with accuracy results. Made a eraser nub and now it is shooting very well. Now I am curious if my problems were more with the omega nub than the macaron bucking. Going to try the macaron with eraser nub sometime after the event tomorrow.

So is there a good concave drop in nub on the market that is worth buying (performs better than eraser nub and reasonable price)? as so far I am hating the omega nub and wouldn't pay 15-20$ for some of those other nubs as they seem too close to the omega. Anyone have a lot of experience with a good aeg nub?

Last edited by Jbrinker1; 11-08-2019 at 06:41 AM.
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Old 11-25-2019, 12:32 PM   #2
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Still haven't got to trying the macaron with my eraser nub.
Quote:
Still a little disappointed with slight velocity drop but I think that might have to do with my stock non-oring nozzle. Any thoughts?
What do you think about this Zero? I have not built a ton of dmrs (a couple for friends) and this is the first I have paid this close attention too as it is my own and from my one-and-only so it has a ton of sentiment. I want it to be near perfect with out spending a ton.
I also thought I saw a post from Zero stating his dislike for oring nozzles and a way to achieve good compression without and oring nozzle. Reasons for dislikes? method for better nozzle fitment?
Mind you right now compression is not horrible but not good at all. For reference, I am using a cut down m130 spring, full cylinder, and 410mm 6.03 prommy. The stock m120 was giving me 380fps w/.20 tops.

Last edited by Jbrinker1; 11-25-2019 at 12:37 PM.
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Old 11-25-2019, 12:47 PM   #3
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Just from my quick scanning of this, I think your inner barrel might be a touch long being an aeg. I don’t think you’ll be loosing too much from the stock nozzle in terms of FPS. If you have other parts kicking around, try fitting in a shorter inner barrel, 330-360mm in length and see your FPS and joule numbers then
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Old 11-25-2019, 03:02 PM   #4
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Just from my quick scanning of this, I think your inner barrel might be a touch long being an aeg. I don’t think you’ll be loosing too much from the stock nozzle in terms of FPS. If you have other parts kicking around, try fitting in a shorter inner barrel, 330-360mm in length and see your FPS and joule numbers then
Thanks for the info. I thought I had it balanced pretty good for 0.30g according to 1tonnes guide. Maybe I am mistaken. I will double check when I get a chance. Now I did have the stock barrel and my velocity numbers were significantly lower (15-25 fps). The stock one is supposed 6.03 and the length is around 75-125mm shorter than the prommy.

What confuses me is the hop up settings significantly affect the velocity negatively. With both weight bbs. Is that normal? And I am not talking about 0 hop to sending them skyward. The adjustment are from 0 to slight over-hop. If it is not my nozzle being perfectly sealed (like with an o-ring), I am a little stumped. Would it be my nozzle to bucking seal? I feel like this is good (defiantly better than stock as I sanded off the front of tappet plate some) but not sure how to check.

I don't remember this being an issue as much on my KA SVD but that has been some time ago since tuning it. May have to get it running again and see what it does.

Last edited by Jbrinker1; 11-25-2019 at 03:06 PM.
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Old 12-05-2019, 03:37 AM   #5
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Do any of you dmr building pros (Zero, Accurate, 1Tonne, Whale, anyone) have any thing to input on this one?
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Old 12-05-2019, 11:28 AM   #6
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Which rifle is this Jbrinker?

A few things here are really sending me alarms. If an M120 was only generating 380fps, the gun isn't right. Same goes for the big drops in fps with hop slightly applied. Air takes the path of least resistance and in this case, it sure is. With a full cylinder and a 6.03 mm barrel, that thing should be rocking over 400 easy.

So, out of curiosity, what's the piston o-ring like? When you put the rifle together, did you hold your thumb over the nozzle tube and try and compress the piston into the cylinder? That's a pretty easy test to see if your piston o-ring is not up to snuff.

As you know, I don't pay much attention to conventional cylinder/barrel ratios...to be honest, I haven't found a whole lot of merit to doing that calculation. If the inner barrel is 509mm, give er! hahahaha. But 410mm is definitely respectable, especially in a 6.03mm prommy.

This gets me wondering about few things...As I said, the piston o-ring. The seal between the cylinder head and the cylinder. Also check the hop up chamber. Was it relatively easy to get the barrel into it with the bucking on? It shouldn't be. You can lose a pile of air through the barrel window. Yeah, I've checked that and it has been a problem on a few rifles. Especially where the hop chamber isn't tight and the window for the arm is bigger than it needs to be. The air will pass around the protruding nub and out the front end of the bucking. That may be why your flat hop is doing better. It's also why I like my beloved r hops. No losses through the window.

In the recent CM I'm so happy with, the fit between barrel, bucking and hop unit is ridiculously tight. Any air going through the nozzle to the bucking is going down the barrel. I'm running an M110, a ported cylinder, a stock 6.08mm barrel, and a stock motor running on a 9.6v nimh. 388 fps, man. You're losing air...

So, first and foremost, check cylinder compression. Also check the cylinder walls for scoring or any way air could bypass.

Let me know how it goes!!
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Old 12-05-2019, 11:41 AM   #7
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A stock KWA KM4 AEG generates 420fps on m120 (at least it used to back in the days). That’s a definition of good sealing and still it can be further improved. If you want a DMR which should be second to no stock gun you need to start with that as a baseline, so if your cylinder-hop-barrel doesn’t give you 420 on m120 everything else does not matter and your DMR will be useless.

Actually after recalling my KWA M4 DMR from years ago, it was shooting 520 on sp130. Use that as your baseline.
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Last edited by Jsmithski; 12-05-2019 at 11:44 AM.
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Old 12-05-2019, 12:37 PM   #8
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520?!?! HOLY SCHNACKERS, BATMAN!!! hahahaha
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Old 12-05-2019, 12:43 PM   #9
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Different spring brands can have slightly different fps rates and yes, an M120 can produce only 380fps. In fact, that is very common. You may have an air leak but you may also not.
Do you have a port in your cylinder? Is the piston plastic or aluminium? Does your sector gear have all of it's pickup teeth (it should)?
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Old 12-05-2019, 12:53 PM   #10
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Quote:
Originally Posted by Jsmithski View Post
Actually after recalling my KWA M4 DMR from years ago, it was shooting 520 on sp130. Use that as your baseline.

SP springs are not the same as M springs. M springs are rated on how many meters per second a 0.2gm bb can travel while and SP spring is rated on how many meters a second a 0.25gm bb can travel. So a SP130 is a 0.25gm bb travelling at 426fps. This is the equivalent of 2.1 joules. 2.1 joules with a 0.2gm is around 476fps (You can get more with a longer barrel). Often when new, springs are running hotter than their actual rating. So before the spring has worn in, it is quite possibly be getting 500-520fps
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Old 12-05-2019, 01:52 PM   #11
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Quote:
Which rifle is this Jbrinker?

A few things here are really sending me alarms. If an M120 was only generating 380fps, the gun isn't right. Same goes for the big drops in fps with hop slightly applied. Air takes the path of least resistance and in this case, it sure is. With a full cylinder and a 6.03 mm barrel, that thing should be rocking over 400 easy.
That is what I was feeling like. This is my Valken alloy series MK11.
Quote:
This gets me wondering about few things...As I said, the piston o-ring. The seal between the cylinder head and the cylinder. Also check the hop up chamber. Was it relatively easy to get the barrel into it with the bucking on? It shouldn't be. You can lose a pile of air through the barrel window. Yeah, I've checked that and it has been a problem on a few rifles. Especially where the hop chamber isn't tight and the window for the arm is bigger than it needs to be. The air will pass around the protruding nub and out the front end of the bucking. That may be why your flat hop is doing better. It's also why I like my beloved r hops. No losses through the window.
Piston seal and cylinder head seal is perfect. I can put my finger on the cylinder head nozzle and press piston forward and hold it tight with no air leaks. Putting the stock nozzle on the cylinder head nozzle and doing the same, I can compress the piston and it take about 3-5 sec for air to all leak out (doesn't feel horrible). The bucking seems to be a pretty tight fit. I did dental floss it as well. I feel like this is pretty good but I will certainly take another look.
Quote:
A stock KWA KM4 AEG generates 420fps on m120 (at least it used to back in the days). That’s a definition of good sealing and still it can be further improved. If you want a DMR which should be second to no stock gun you need to start with that as a baseline, so if your cylinder-hop-barrel doesn’t give you 420 on m120 everything else does not matter and your DMR will be useless.

Actually after recalling my KWA M4 DMR from years ago, it was shooting 520 on sp130. Use that as your baseline.
That is what I am feeling like. Even now it out performs most AEGs on the field but nowhere near what I feel like it should be. 520? Wow nice!
Quote:
Different spring brands can have slightly different fps rates and yes, an M120 can produce only 380fps. In fact, that is very common. You may have an air leak but you may also not.
Do you have a port in your cylinder? Is the piston plastic or aluminium? Does your sector gear have all of it's pickup teeth (it should)?
I am not 100% confident exactly what my spring is as I got a few projects on the bench but am 90%. I have a full cylinder, no port. The piston is the stock plastic with metal rack and the sector gear has all the teeth.
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Old 12-05-2019, 02:25 PM   #12
 
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I would look at your nozzle to bucking seal. I do not think its seating well.

Have you shimmed your gearbox in the body to adjust your nozzle alignment? What hardness is the ML bucking? Have you trimmed your motor grip so it does not pull your gearbox when tightened to the body? What hopup chamber are you using? You are teflon taping the bucking as well as the dental floss? With the bucking on the barrel, can you drop a bb threw the bucking or does it need a little force?
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Old 12-06-2019, 01:59 AM   #13
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Quote:
Originally Posted by 1tonne View Post
Quote:
Originally Posted by Jsmithski View Post
Actually after recalling my KWA M4 DMR from years ago, it was shooting 520 on sp130. Use that as your baseline.

SP springs are not the same as M springs. M springs are rated on how many meters per second a 0.2gm bb can travel while and SP spring is rated on how many meters a second a 0.25gm bb can travel. So a SP130 is a 0.25gm bb travelling at 426fps. This is the equivalent of 2.1 joules. 2.1 joules with a 0.2gm is around 476fps (You can get more with a longer barrel). Often when new, springs are running hotter than their actual rating. So before the spring has worn in, it is quite possibly be getting 500-520fps
True, the spring was labeled sp130 (and brand was Systema if I remember correctly) but it did not wore out, after a year it was still within 520 range). That gun was something I’ve never build again, my last AEG project.
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Old 12-06-2019, 07:13 AM   #14
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Quote:
I would look at your nozzle to bucking seal. I do not think its seating well.
Makes sense. I am going to put the stock bucking back in as maybe my g&g green and ML just dosen't play well with the stock nozzle. If there is no difference, then I am going to make sure there is no leaks between the bucking and barrel as Zero was saying and just eliminate this from the equation. If that doesn't work, then it has to be an alignment issue like you are saying.
Quote:
Have you shimmed your gearbox in the body to adjust your nozzle alignment? What hardness is the ML bucking? Have you trimmed your motor grip so it does not pull your gearbox when tightened to the body? What hopup chamber are you using? You are teflon taping the bucking as well as the dental floss? With the bucking on the barrel, can you drop a bb threw the bucking or does it need a little force?
I am not following you a 100% on the gearbox shimming as it is pinned to the receiver pretty tight. I am not sure how much room, if any ,there is to shim. Which axis do you think it could be off? The ML is a 60deg I think. Might be a 70. Not sure what g&g greens are or what the stock is (stock seems to be the harder of the three). I have not trimmed the motor grip as it is the stock grip with stock gearbox with stock receiver. I assumed they should all fit right together, correct? I am using the metal lonex chamber. I did not notice a significant difference between it and the stock plastic one. I did not teflon the bucking, just dental floss. I am going to have to try the bb drop test. What should I be looking for/seeing here on that test and what does it tell me?
Thanks guys for all the input. I have a new vigor for this project. Hope to work on it some this weekend.
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Old 12-06-2019, 11:02 AM   #15
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Vigor is good! As long as it doesn't become an all consuming quest into madness. hee hee!

The drop test is when the hop up and barrel are assembled together and you hold it vertically with the barrel pointing down towards the floor. Drop a BB into the hop up and see if it sits on top of the bucking lip inside the hop unit. If it just drops through, you know the bucking isn't tight enough. This is a great starting reference but keep in mind that under load, all these conditions change.

Oh! Also check to see that when the bucking is installed into the hop up unit that the lip isn't beinding on anything and is perfectly seated round into the hop up unit. Any deformation will cause a big leak.
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