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GSO-Geko

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

 

http://arniesairsoft.co.uk/forums/index.php?/topic/210618-tokyo-marui-m870-tactical-gas-tri-shot-shotgun-picture-text-and-video-review/?hl=%20marui%20%20m870

 

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.

 

Disassemble

https://www.youtube.com/watch?v=p3zD8GL9VBs

 

Reassemble

https://www.youtube.com/watch?v=xNsdjilxly0

 

Trouble Spots

https://www.youtube.com/watch?v=F7u4qRz4-Zo

 

 

Some tuning Advice

 

To Increase FPS:

DSC_0551.jpg

 

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).

 

DSC_0560.jpg

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.

DSC_0529.jpg

 

Take this thing apart and find this bit.

DSC_0526.jpg

 

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.

DSC_0544.jpg

 

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.

DSC_0547.jpg

DSC_0550.jpg

 

Cut a little bit of that nozzle back

DSC_0564.jpg

 

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

DSC_0568.jpg

 

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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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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LOST BARREL BRACKET FIX

 

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.

DSC_0626.jpg

 

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,

 

DSC_0636.jpg

 

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.  

DSC_0628.jpg

 

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. 

DSC_0630.jpg

DSC_0631.jpg

 

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.

DSC_0633.jpg

DSC_0634.jpg

 

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.

DSC_0664.jpg

And so workbench time.

DSC_0663.jpg

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

DSC_0663.jpg
DSC_0666.jpg

Guide hole time.

DSC_0667.jpg
DSC_0668.jpg

Slowly working my way up the bit size to completely remove the last post stump.
DSC_0669.jpg
DSC_0670.jpg

I used a 4mm screw with the head trimmed off.
DSC_0672.jpg

Boom. Fixed.
DSC_0673.jpg

  • Like 4

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Question for you guys if I may! My Breacher has recently stopped working on 6 shot. It now only first 3 unfortunately! Can someone give me a hand to get it back up to speed? Could it be the selector spring? 

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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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No they arent. I don't think they wouldve warped already though, the breacher has only done 10 shells ish I would say! If its that I've got a nebula plastic reinforced loading nozzle as spare. 

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