PMAG's first 20rds huge drop in FPS

[TANNER]

Hey guys

I'm having a strange problem with my Magpul mags, Both the PMAG & TMAG models. When loaded to their max of 120rds the first 20 or so shots suffer from a dramatic drop in FPS (around 150fps), after that they seem to stabilize. If I instead under-load the mag to 90 - 100 rds the first few shots may be a little lower in FPS but it will stabilize after 1-2 shots.

Gun is a VFC PDW, upgrades include:

Lonex high torque motor,
Lonex 16:1 gears,
Lonex piston head with SHS piston,
SHS M120 Spring
Sorbo pad
G&P hopup (VFC wasn't feeding properly with the PMAGS)
Mosfet.
Running a 7.4v lipo.
(The Cylinder head + Nozzle remain stock and are both plastic)

Did some tests shown below, this happens pretty consistently. Tested with .28g using the .25g setting on the chrono, gun usually shoots around 385 fps on .20 (didn't have any .20 lying around to test with).

Video results: http://www.youtube.com/watch?v=YpyRR...ature=youtu.be

---------Mag loaded to 120 rds.
208
348
196
355
189
356
201
210
215
349
210
213
216
356
207
357
204
352
350
340
315
--------- Stable FPS begins after around 20 shots, from then on out it is fine.
359
353
354
214
335
348
355
355
352
346
357
348
360
351


Does anyone have an insight as to why this is happening? Could it be the spring in the mag is too strong causing the nozzle to be pushed up out of place?

Appreciate the help,
- Chris

[MaciekA]

Adding to this thread because I have personally observed ChrisUK's gun in action several times and want to head off some people's usual suggestions off at the pass.

As you can see, after 20 shots this gun has stable and fine compression. I've looked at the piston o-ring, the cylinder head seal, etc, and it all comes off as fine.

Here's the catch though:

These same mags cause the exact same issue in two of my own AEGs -- one a PDW and the other a SIG 556.

My personal theory is that the mag spring is so strong that it pushes the nozzle upwards enough to cause a small leak. This is supported somewhat by the fact that the SIG seems especially sensitive to this effect -- after all, it has a nozzle that is something like 3 times the length of a typical v2 M4 nozzle.

I'm also all ears to ideas from people who have actually encountered this issue and solved it.

[XZIVR]

I was thinking the same thing regarding the bbs pushing on the nozzle. Especially with a different hopup. I would try replacing the nozzle. With a good oring type. It's a relatively cheap thing to do and a good upgrade anyways

[MaciekA]

Originally Posted by XZIVR

I was thinking the same thing regarding the bbs pushing on the nozzle. Especially with a different hopup. I would try replacing the nozzle. With a good oring type. It's a relatively cheap thing to do and a good upgrade anyways

Sadly this knocks the KA SIG 556 out of the running, unless someone out there has made a super long nozzle that can avoid this problem

[XZIVR]

It wouldn't happen to be the same as a masada nozzle would it? Hmm i should try a pmag in mine and see what happens..

[TANNER]

I have a SHS metal nozzle lying around that I am going to swap in and try. Will post results.

[lurkingknight]

I have reason to believe I get this on my CA g36 as well using g36 pmags... first 10-20 come pretty much rolling out the barrel.

I'm not sure on the exact round count, I should probably take some care and actually count out how many are going in. I'm curious to know if I'm actually overfilling beyond 120 or if there's something going on.

Also, on my KS p90 gearbox it would do this with midcaps, though the airseal on that gun was inexplicably bad no matter what I did with it. But shots would stabilize at about a half load in a midcap.

[TANNER]

Swapped in the SHS nozzle and the results were even worse, first 20 shots were consistently under 200fps. It did eventually stabalize but with a slightly lower fps

Hmmm

[MaciekA]

I think this is a flaw in the Marui design, and requires a fix akin to the STS (shock transfer system) or similar, where we fill gaps in the design to prevent the motion of certain parts in and around the gearbox.

The nozzle should have a guide ridge on top with a guide rail so that it isn't able to move even 0.1mm off course. I am fairly certain this is what Marui was doing when they designed a G36 nozzle that looks like this:



One other way to fix this is to find a nozzle that has a perfectly cylindrical exterior and guide it along a halfpipe or something so that it can't travel around.

Another way to possibly fix it would be to guide the end-tapering of the nozzle into position with a guiding torus/ring that slams it into the correct position and doesn't let it move due to the pressure from BBs below.

That, or you only allow BBs to travel upwards towards the nozzle if the tappet plate reverses. Right now, the flaw in Marui's design is that the nozzle is the trap door, and yet we expect it to magically have an absolutely precise seal yet the only thing holding it in place is the corner tab of the tappet plate and a squishy piece of rubber at the bucking. Can you imagine how many bugs are being caused by this interaction?

One final note: There are dozens and dozens of M4 nozzles out there on sale, with a huge variety of taper shapes on the end and small variations in overall nozzle length. I would be willing to bet some of those are less willing to shift from the BB push than others -- I'm thinking a slightly longer (~0.1mm longer) and slightly less-tapered (more square edge than diagonal edge) nozzle will solve this problem.

[Stealth]

Any option to use a different cylinder head with a longer "spout" which would help to stabilize the nozzle?

[TANNER]

@lurkingknight did a round count and was not overfilling mine, 120 rounds exactly.

@stealth are you asking if I have another cylinder head to try? If so I do not at this time, but I may grab the lonex version from you next week.

[TANNER]

One thing I did observe is that my hop unit does have alittle play when in place, the wings that hold it in place look alittle narrower than the VFC hopup. Could this play be contributing?

[MaciekA]

Originally Posted by ChrisUK

One thing I did observe is that my hop unit does have alittle play when in place, the wings that hold it in place look alittle narrower than the VFC hopup. Could this play be contributing?

Reducing the play probably wouldn't hurt, but the question is whether it's starting too high or too low. How do you calibrate this so that there's a straight plumb line from your spring guide all the way to the center of the end of the barrel?

[lurkingknight]

it's hard to prove that the nozzle is losing compression when the mag spring is pushing up against it.. I was trying to prove this on my p90 to come with a reason why it was 50fps light and not shoting/feeding properly with the midcaps.

When I got my new p90, I tested pushing the nozzle around and it too lost compression with about the same amount of force was put on it, but it also fed perfectly from midcaps at the correct fps. Even when I swapped out the cylinder head and nozzle from the suspect gearbox into the good one, the new gun shot just fine, and chronoed the same. So I couldn't exactly prove the leak was there when the mag was full.

I'm still not quite sure if it's consistent with issue I'm having with the pmags and g36 though, I have not really gone over it in that much detail with the g36, it was more of an observation recently than an active attempt at fixing it, since my g36 is shooting 400 fps now as it should be, yet still rolling BBs out with really full mags.

If it's any help, I have a lonex v3 CH, a lonex g36 nozzle and CA hopop.

One point of note though, the g36 cylinder head IS longer than standard v3. so maybe I should be trying the lonex AUG CH. Not that I want to... it shoots fine right now... if I loose 10 shots per mag, I will just get 1 more mag if it's that big of a deal. It's 30$ and no time vs 15$ and lots of time to troubleshoot, with no guarantee that I can fix it.

I'll do a more accurate count/load next time I'm at the field though, just to verify if I'm overloadaing the mags or not.

[coach]

Short load your mags to 100 and call it a day!