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GSO-Geko

Marui M870 Fix and Tuning Thread

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No problems with mine. Took me some time to get some mods to work correctly but that's just airsoft :) No breakdowns or disappointments so far.

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My first M870 tactical I broke a few things after 7 months of very hard use. I put maybe 400-500 shots a week through it. Based off that, the are really solid. If you know how to take care of it of course. I've had years experience with the old marui tri shot system, so I'm no stranger to being cautious of breaking nozzles.

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I've recently bought a breacher, had it for about 2 months before I got any gas to test it, finally had a go this weekend and as soon as I load the mag the gas just escapes out of the body of the gun. Never fired a shot, currently it's a £300 paper weight  :angry22: I gas the mag and as soon as it locks into place I hear gas escaping inside, empting the mag in less than 2 seconds.

I've heard people say they leak if you use Green gas/ Propane but I've not actually ever used it, any advice would be much appreciated.

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Does anyone have the O Ring size for the 3 o rings that the screws go through to hold the expansion chamber together? Mine became damaged and I've had issues trying to find replacements.

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If I were to replace the inner barrels of an M870, how long could they be?

I'm not an expert on gas guns, but with AEG there's a limit on the lenght of the inner barrel based on the cylinder window position (volume of compressed air).

But with gas guns I have no clue on how it affects (if at all).

 

Anyone can share some info? Please? :)

Edited by Rabianegra

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Gas expands more than air and thus while you have a 'cylinder' reserve of air and need to fill your barrel with that reserve, gas just keeps expanding.

That's why the longer the barrel, the higher the exit speed of the BB. 

Read: it depends on what your field limit (FPS) is.

 

My explanation isn't completely correct, but it gets you there.

Hint: Laylax makes precision barrels for M870, but are expensive.

Try google them and perhaps you'll find their length?

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Laylax are the same lenght as the original. 

My goal is to get around 330 fps with 134a. Ideally, longer range while keeping original dispersion.

But I don't know if the gas expanding too much will actually slow down the BB. (Don't know if it makes any sense either :D)

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Why would gas expanding too much slow a bb if it's what's pushing it to begin with? With gas guns there generally is a whole lot more gas expelled than pneumatic guns so the allowable barrel lengths are longer before they start being counter productive. As for what exact length that is, it's hard to tell but suffice to say anything that will be covered by the outer barrel will still be fine.

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Thanks renegadecow.

Based on my rudimentary physics, I'd imagined that the longer a gas expands, the slower the expansion rate will be. And, as the BB is being pushed by the expanding gas, I thought it may be slowed down if the barrel was too long.

 

But if you say that anything shorter than outer barrel is fine, perfect for me. Will see how much/if range and speed increase if I finally do the upgrade.

 

Salamat sir!

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I DID IT! I BLOODY FINALLY DID IT!

 

Both tactical and breacher holding green gas with the Angry Gun stock in my temps (28°C-30°C). The tactical shooting 340fps with .25g in 3 shot mode and the breacher doing 310fps. I'll be following this up with pics but basically I made a pinhole restrictor for both so that the expansion chamber in the gun doesn't get flooded with liquid propellant which is what causes the dreaded leak.

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I DID IT! I BLOODY FINALLY DID IT!

 

Both tactical and breacher holding green gas with the Angry Gun stock in my temps (28°C-30°C). The tactical shooting 340fps with .25g in 3 shot mode and the breacher doing 310fps. I'll be following this up with pics but basically I made a pinhole restrictor for both so that the expansion chamber in the gun doesn't get flooded with liquid propellant which is what causes the dreaded leak.

Hooorah!

 

You dont happen to know which generation of m870 you have do you

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I believe they're the latest version as they both came with steel reinforced loading arms.

 

As for the miracle working thingamawhatsit, this is it along with an o-ring which is used as a spacer. Looking like I stole it off a set of Mastermind, I turned it out of ABS plastic with a ghetto lathe (drill). It goes between the receiver and Angry Gun stock to restrict gas flow which I calculated from opening up the valve on the stock TM magazine which sort of acts as a flow restrictor and probably also  why most people using the stock components don't seem to have any leaking problems. Mind, I've also stretched and tempered the main valve spring in the gun to help it close better which I'm guessing is also necessary for this part to work. I'll post dimensions shortly.

IMG_7321.jpg

IMG_7320.jpg

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Right! Almost forgot about that. Dimensions (not to scale) below.

flow%20restrictor.jpg

And the o-ring is 2mm(CS) x 4mm (ID). I have to say that it doesn't make it technically 100% leak free. After about 2-3 refills of gas I found that it eventually started to leak again but very minimally. Because I was trying to emulate whatever physical characteristics the stock components had, the only thing left different is the removable gas tank. Then that's when it hit me: every time you fill up the tank you need to remove it which effectively purges the expansion chamber, filling it with ambient air. I can't even come close to explaining the mechanics of it except that the expansion chamber needs this pocket of air to work properly. So I emptied the Angry Gun tank and proceeded to remove the fill valve. Either by leaving it alone for a couple minutes or blowing into the reservoir (both work), the gun is purged of propane then closed again before refilling with green gas. The result: leak free until that air pocket is depleted some 2-3 reloads after. 

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Interesting, makes me question the design of TM  especially since they just released news of their wood m870, which is a heretic act within itself. Then again they'll justify issues since we're now using duster which just int powerful enough.

 

I am a little lost why this "air pocket" is essential, i figure a vacuum space with the needed gas is preferred, not to mention allowing more shots. Is it safe to say it'll always leak with GG or propane? What about the PRO ARMS/SAT valve??

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Mind, that's 2-3 reloads of gas, not shells which is around 100-150 shots worth. Well enough to go through a weekend of shooting without worry and more than enough before getting Popeye arms. I can't really say myself why the air pocket is essential as the pressure in theory will still peak once the whole system warms up with or without it. For one thing it prevents the over filling of the gun, but again, I can't see how that affects anything except that it's the big difference between it leaking or not at least from my findings. With the SAT valve, they themselves say it's incompatible with the Angry Gun stock. Their solution is to completely disassemble the gun and relube all seals which I think all it really does is allow that air pocket back in before reassembly.

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All reports i have read show that the SAT valve is expensive junk. Regardless of lubing seals or using the stock set up it leaks horridly. I have yet to find 1 account of a good review. ALthough i can not find info on the pro arms variation that is negative or positive

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I'm looking at an 870 for someone who broke their nozzle & hop rubber... As far as I can tell as it loads the BBs into the top barrel it folds the hop rubber over & jams in place, thus taking out the hop rubber (if it's forced) and the nozzle behind. I can't find any information on this fault anywhere online except a forum I can't get onto...

Anyone got any ideas? There doesn't seem to be any wear around the loading area, the loading arm's been replaced as has the nozzle & hop rubber, but I'm guessing it's still got the same problem that took the parts out in the first place... I'm pretty sure I'm not the first person inside this shotty, but there doesn't seem to be anything obviously miss-assembled.

Any help would be appreciated thanks folks!

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Reading this thread I get the impression this shotgun breaks down a lot. Is that an accurate impression? Makes me worried about getting one.

 

 

Sent from my iPad using Tapatalk

My tac leaked, never fed a bb, was more wobbly than a wet noodle. Edited by Alias1983

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Having an issue with my breacher, when set to 6 shots, the pump jams and puts itself back to 3 shot mode, on 3 shot mode it jams also but not 100% of the time like 6 shot mode. It seems that it jams by trying to load to many bb's. Would this be a replacement nozzle job?

Edited by fabio

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