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Marui M870 Fix and Tuning Thread

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A lot of advice and stuff has been offered on the m870 thread seen here




However, since the thing is now 30 pages long, I felt it necessary to create a thread specifically to collect all technical advice related to the gun.


For starters, my three videos related to the gun.








Trouble Spots




Some tuning Advice


To Increase FPS:



A small Oring on the bottom of the hammer spring will increase the FPS by approximately 30 (or 10 mps if metric is your thing).



Polishing the hammer rails will allow the hammer to slide forward much faster and smoother.  No velocity change measured, but it can't hurt.


If you're getting a consistent slow leak from the body.

Most of the time the culprit is the expansion chamber.



Take this thing apart and find this bit.



I've found that the orings here are often dry.  Means the valve won't close completely and cause a leak.  Over time, this will completely ruin these orings.  A little bit of silicon oil fixes this quite easily.  For good measure, apply a bit of silicon to all the orings in the whole thing.


Broken nozzle fix.  


This was a rough try at this, results are promising, maybe someone with more machining skills can do a better job.


I noticed the top nozzle on my was totally fractured, rendered the gun unusable.



So, a bit of 4 mm brass tube cut to length will provide support.

I did have to drill out the nozzle a bit to fit the brass tube.




Cut a little bit of that nozzle back



And a bit of 5mm brass tube over that to replace the bit you cut back



Make absolutely sure that the new "nozzle" is the correct length.  If it's incorrect you'll get feeding problems and the gun won't be able to rack properly.


As people ask questions and find solutions, I'll add things to this first post.

Edited by GSO-Geko
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A lot of advice and stuff has been offered on the m870 thread seen here   http://arniesairsoft.co.uk/forums/index.php?/topic/210618-tokyo-marui-m870-tactical-gas-tri-shot-shotgun-picture-text-and-vi

Loading Arm Fix   So my feed arms finally broke... kinda.  The posts on either side of it are bent down slightly, this means that the loading arm doesn't get pushed down enough to allow bbs to feed.

Just to echo what others have said.   My breacher is fffffiiiiiiiiiiiinnnnnnnnnneeeeee. Damn fine in fact. Sexy fine. Gggggrrrrrr fine.

Kinda. I run it on HPA at the same pressure as green gas.


I can get around 350 FPS with 0.25s.


I was also under the assumption that, like my regional forum, I would be able to edit any of mine own posts for any length of time.  Sadly, this is not the case.

Edited by GSO-Geko
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So if you're like me, you use this gun a lot.  And during use sometimes the black plastic bracket at the end of the inner barrels will fall off.  And sometimes you will notice and find it, other time (non-plural) you will not.  This is more of a problem with the SAT brand shorter barrel, as I think the internal dimensions are slightly wider.


However, electrical tape fixes everything.


First attempt was with a 4 layer think wrap around all three barrels.



This made it so there was virtually no spread on the bbs out to 100  feet, or around 32 meters.  While this allowed me to live out my dream of being like those guys that get around sniper limit servers on Battlefield 4,




I needed the spread back.


Thankfully with calipers and careful cutting you can wrap a 49 mm length of electrical tape around a single barrel almost exactly twice.  



This put the spread right back where it should be.  And has just enough pressure against the inner wall of the outer barrel to keep them stable.


Putting rails on the stock foregrip


I purchased a G&P railed foregrip, but I found it to be too heavy and loose.  Therefore, a rail on the stock grip became the best option.


Using a G36 rail set I bought on ebay, first came attaching the rail sections for measurement and pilot hole marking with a silver sharpie. 




With the pilot holes drilled, I widened them out to accept an M3 screw.  And countersunk the inner surface of the foregrip to accept the screw heads.




And done.  It's very sturdy and easy to do.  Plus light enough not to ruin the run-around-all-day with it vibe.  Picture above of full shotgun displays the finished result, and no, that's not how I run it.

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Loading Arm Fix


So my feed arms finally broke... kinda.  The posts on either side of it are bent down slightly, this means that the loading arm doesn't get pushed down enough to allow bbs to feed.


And so workbench time.


First step is to remove the posts themselves, but not completely, we need a location for the guide hole.


Guide hole time.


Slowly working my way up the bit size to completely remove the last post stump.

I used a 4mm screw with the head trimmed off.

Boom. Fixed.

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Not 100% positive on opening it up unfortunately.. did it once and managed to bend the loading arms, so am pretty hesitant now. Does anyone have an idea of possible problems before I go at it and crack it open? It just went from firing 6 to 3 over the space of one shell pretty much. 

Edited by DeltaZero
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