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KM4 SR-5 FPS issues

sr5 km4 barrel loss of fps cylinder

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#1 carbinenewb

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Posted 07 February 2014 - 10:18 PM

Hey Guys,

I am new to this forum and have an old sr5 that I bought 2nd hand from my local retailer. The gun itself has never been gamed, barely shot and basically mint for a 2009. I chrono'd it recently bone stock and got about 330 fps on a .20. After some mods: lonex ported piston head, shs poly w/ steel gear piston, sorbo padding, 110 spring, reshimmed, lubricated, shs high torque motor and mosfet, the gun fps'd at 350 on .2g . The gears and most internals are stock except for those listed above...I did these for durabilities sake. I did my research and my cylinder is ported all the way to the rear which means the gb is a low serial number sr5. The aoe has been redone and pre engagement has been eliminated as a result of the 2009 stock spring being replaced with a 110 and lightened piston head assembly. An important fact is that we had a sr10 upper reciever with a stock barrel and we placed my sr5 lower reciever for testing....we got 400 fps on the dot... so we know that the gb is actually shooting ok..(could be better with oring nozzle and hop up bucking). So my question is...Do i need to look at the cylinder porting? do i need a tight bore? Ive had my fair share of airsoft back in 2008 then i went awol and now im back :)

From all my research it seems to be that i need a different version of cylinder apparently the newer sr5's had that. Its funny to think that my stock sr5 when placed on a 360 + mm barrel will actually shoot 400 fps. my guess is that the point at which the piston is about to produce maximum psi, the round has already exited the  227mm barrel.

Thanks in advance for the assitance!

Edited by carbinenewb, 07 February 2014 - 10:28 PM.


#2 Larry

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Posted 08 February 2014 - 07:59 AM

Sounds like your issue is in the upper, barrel/bucking fit in hopup assy. Spring missing that pushes hop into reciever assy. Something in that area. Barrel may account for 10 or 15 fps i would think, depending on bore. I have seen different buckings make a huge difference in fps, make sure it is installed correctly.

#3 gcw360

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Posted 08 February 2014 - 08:47 AM

View Postcarbinenewb, on 07 February 2014 - 10:18 PM, said:

Hey Guys,

I am new to this forum and have an old sr5 that I bought 2nd hand from my local retailer. The gun itself has never been gamed, barely shot and basically mint for a 2009. I chrono'd it recently bone stock and got about 330 fps on a .20. After some mods: lonex ported piston head, shs poly w/ steel gear piston, sorbo padding, 110 spring, reshimmed, lubricated, shs high torque motor and mosfet, the gun fps'd at 350 on .2g . The gears and most internals are stock except for those listed above...I did these for durabilities sake. I did my research and my cylinder is ported all the way to the rear which means the gb is a low serial number sr5. The aoe has been redone and pre engagement has been eliminated as a result of the 2009 stock spring being replaced with a 110 and lightened piston head assembly. An important fact is that we had a sr10 upper reciever with a stock barrel and we placed my sr5 lower reciever for testing....we got 400 fps on the dot... so we know that the gb is actually shooting ok..(could be better with oring nozzle and hop up bucking). So my question is...Do i need to look at the cylinder porting? do i need a tight bore? Ive had my fair share of airsoft back in 2008 then i went awol and now im back :)

From all my research it seems to be that i need a different version of cylinder apparently the newer sr5's had that. Its funny to think that my stock sr5 when placed on a 360 + mm barrel will actually shoot 400 fps. my guess is that the point at which the piston is about to produce maximum psi, the round has already exited the  227mm barrel.

Thanks in advance for the assitance!
Hi Carbinenewb - Welcome to the KWA Forums and congratulations on your SR5.  That is my favorite CQB AEG.  In fact, I have two each with serial numbers in the 30's so I know exactly what you are dealing with.

Stock (first run) SR5's came with a KWA tight bore (yes tight bore. It wasn't heavily promoted as such), a Type 0.5 cylinder, and some grade of KWA spring that was apparently an m120.  They initially fired 330-335 settled.  By simply changing the gun to a type 1 cylinder the FPS would shoot up to 365 FPS.  Adding a carbine length barrel brought you into the 390's.  An SR10 length barrel and the original 0.5 cylinder got you 400 FPS.

So regarding you questions on TBB and the cylinder....that depends on how you will use the gun.  Swapping the cylinder should boost the FPS with the short barrel that you currently have.  A TBB of the same length shouldn't do much for performance, especially if you are playing CQB.

The CQB field limits around me are 350, so I can play right at the limit with my SR5 now after all these years.  It has the type 1 cylinder, and when tested outdoors is very effective up to 150ft.  It reaches beyond that, but I would much rather my KM4A1 when outdoors.

Hope this helps.

#4 carbinenewb

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Posted 08 February 2014 - 02:17 PM

Thanks for the quick replies Larry and gcw

Larry i did change up the bucking with the working sr10 bucking(which yielded 400 fps) and still produced exactly the same fps of 350. so i ruled out the bucking and the seal between bucking and nozzle.

For Gcw, Total appreciate the run down on the sr5! I knew it had something to do with the cylinder. I will try a type one as soon as i can. And feed some results.
I did however stumble across the cqb edition which had the ports at the center of the cylinder(which were very exaggerated in size) perhaps getting this, supposedly mod 3 or 4?, cylinder would do the trick for perfect compression. My goal is 400 @.2 g. I would like to game it out on the field..i know its not milsim'esque but im using it for portability.

would this be the cylinder? http://store.kwausa....ck-1-5-4-1.aspx
and is this the type you purchased gcw?

thank you guys for the responses!

#5 carbinenewb

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Posted 08 February 2014 - 02:38 PM

found a post from you gcw! http://www.kwausa.co...pic=11798  this might work ;)





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