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KM4 RIS with KM4 CQB gearbox


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#1 choochoo

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Posted 23 September 2013 - 01:49 PM

I got this gun used (I should have known better....)  Before "original" gearbox on the KM4 RIS cracked apart, the gun was shooting around 350-360fps on 0.25gm bbs and 390-405fps on 0.20gm bbs with the following modifications:
M110 madbull spring, 300mm 6.03mm madbull inner barrel, what looks like an aftermarket piston and cylinder head. Everything else looks stock, but can't tell.

I took a risk and got a "low mileage", "almost new" KM4 CQB 2GX gearbox off someone with the only modifications being a Prometheus non-linear M120 spring.  I haven't received it yet.  So, I don't know what the internals look like, but he said everything is stock except for the spring and only 500bbs through it.

Anyways, to make a long story longer, I'd like to use a longer 6.03mm inner barrel 363mm or 407mm and use either the madbull M110 or a M100 spring.  I'll either get a new KWA bucking or get the Lonex, which seems to be the consensus for being a good hopup bucking upgrade.  This gun is going to be for field play.

Here are my questions:
My guess is that the cylinder in the CQB gearbox is is different (lower volume?  part number different in the kwa proshop).  Should I put the RIS cylinder in the CQB gear box?  Or is the KM4 CQB gearbox fine as is?

Could a M100 spring give me 350+ fps with a longer tight inner barrel?  Can the M100 spring perform with the longer inner barrel?  Or should I just put the Madbull M110 in the new gearbox or keep the prometheus M120 in there?  400fps is the limit of most fields here.

I currently have a 11.1V 1300mah lipo, would downgrading to a 7.4V get me gearbox greater longevity?

Oh and I'll be installing a HH airsoft mosfet into the "new" gearbox.

Sorry, for the 20 questions.  Any help for this newb is greatly apprectiated.  this is my first AEG.  Looks like I would have been better off buying a CQR :)  oh well, you live and learn.

#2 Chuck S

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Posted 24 September 2013 - 02:26 AM

My KM4-CQB shoots just under 350fps so you're going to need a heavier spring and larger capacity cylinder to push to 400fps.  To my knowledge cylinders are optimized to barrel length.  Heavier spring can push the BB faster but if there's not enough volume of air behind the BB a longer barrel drags the velocity down due to barrel friction.

The last thing I'd do is invest in a $100 inner barrel.  Springs and cylinders are much less expensive if you have the skills to open and close the gearbox.

You can buy a complete custom built gearbox in the Proshop.

-- Chuck

#3 choochoo

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Posted 25 September 2013 - 02:29 PM

OK, I think i'll swap over the cylinder from the RIS into the CQB gearbox.

#4 choochoo

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Posted 03 October 2013 - 02:44 PM

I ended up getting the Madbull 6.03mm 407mm long inner barrel.  I put the RIS cylinder and piston into the CQB gearbox, switched the M120 Prometheus for the Madbull M110.  Getting the same FPS as before, maybe an ever so slight bump about 358-370fps on 0.25gm bbs.  I'll have to test with some 0.2gm bbs, but I think I'm slightly higher than the 400fps limit at most fields here.

On another note, I replaced the KWA stock motor with a Lonex A1.  So much snappier!




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