PMAG's first 20rds huge drop in FPS

[TANNER]

@coach where is the fun in that?

[MaciekA]

Well...

I've been working on this problem pretty much non-stop since we started this thread and I'm starting to make progress.

Nozzle lift due to mag spring pressure seems to be partially curable by ensuring a tighter and more aggressive seal of the nozzle into the bucking. This could create a feeding problem, so have your sector delayers at the ready if necessary.

One nice thing about the KA SIG 556 is that you can work on it with the "hood open", as it were, which helps the debugging process and giving hints as to what to try next. Check this craziness out:



As you can see, where in a typical V2 M4 setup, the hopup would mate directly with the gearbox, in this SIG it actually sits like an inch away from the gearbox. This necessitates the involvement of a "bridge" to hold the hopup unit in the correct position. The bridge is the thing between the gearbox and the hopup, and it's made of plastic.

In the picture below, look at the gap between the bridge and the gearbox:



My work has focused on closing that gap with successive modifications. I'm risking causing a feeding issue, but I've more or less proven out that if I can shorten this bridge by a very slight amount, I can get the SIG's gigantically-long nozzle to make a better seal against the bucking, which my current testing suggests will cure the problem for my SIG. In Chris's case the cure seems to be somewhat similar, perhaps he will post on that separately.

[Stealth]

I just realized that I'm having the same issue with my KA SR-15 E3 carbine.

Setup is:
G&P hop-up unit.
SHS Nozzle
KA cylinder head
PDI W Hold rubber


I never noticed it during testing because I never load the mags to the top but this past weekend, the first 10 shots out of a full mag would flop, then stabilize afterwards.

I'm going to swap to another unit to try my luck.

[MaciekA]

Originally Posted by Stealth

I just realized that I'm having the same issue with my KA SR-15 E3 carbine.

Setup is:
G&P hop-up unit.
SHS Nozzle
KA cylinder head
PDI W Hold rubber


I never noticed it during testing because I never load the mags to the top but this past weekend, the first 10 shots out of a full mag would flop, then stabilize afterwards.

I'm going to swap to another unit to try my luck.

Since you bumped, I'll go ahead and update my side of things...

The theory with my SIG didn't pan out, and I tried a whole crapton of other things. I even modified other hopup units to accept the hopup-to-gearbox plastic bridge and none of them made a difference. I tried other buckings, etc. Nothin'!. I thought I was getting close, but it was a mirage. The problem would come back again.

My current theory is that in some cases the nozzle is actually under enough pressure that it is unable to return back to the forward position fast enough before the piston starts pushing air through the air seal nozzle. I'm not sure what to do next. If I had access to the right resources, I'd probably try to redesign the way the system works altogether. The usage of the nozzle for two purposes is a critical flaw in Marui's designs IMO.

ChrisUK and I played a game at Finch's this weekend and he described his solution as "take it apart and put it back together with the original bucking/etc again". The result just works.

My theory with his build is that the bucking's protruding lips must be in the correct position/form in order to form a correct seal, and sometimes the lips are mis-configured after insertion into the hop unit. This can be tricky if the bucking's fitment over the barrel is tight *and* you've shoved it into the hopup unit with any reasonable amount of force, thus slightly pulling it back onto the barrel and potentially receding those seal lips away from the nozzle's end.

[TANNER]

@stealth

I didn't get a chance to properly test the Lonex gear as I didn't have much luck getting any of it to work. The Lonex rubbers (all 3) didn't seem to like the Lonex hopup (strange) or the G&P & VFC units I have, kept jamming. As the game was approaching I decided to put back the original tested setup and by some magic the problem fixed itself.

Current setup: G&P hopup, VFC rubber & barrel wrapped with teflon tape.

[MaciekA]

Continuing this discussion at ASM where I'm hoping there will be some people who can give us hints on this issue:

http://forums.airsoftmechanics.com/i...ic,8409.0.html

[BennyBoy]

sorry if I missed this, is it only with those mags? does it work fine with other mags?