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Lower FPS on new upper.

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#1 Migwl300



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Posted 08 August 2014 - 03:10 PM

I tried building a SR-15 upper for outdoor play so the fps was supposed to be above 350 with .20.  It shoots about 260-270 with a .25 so that's way too low.
Parts List
  • RA-TECH 6.01
  • Dytac 16 inch mid-length out barrel
  • URX rail ( forgot manufacture
  • KWA upper receiver
  • EVO II Tension arms
  • RA-Tech hop up unit(everything besides tension arm)
  • all the other according OEM kwa parts besides the dust cover and forward assist
The outer barrel was too tight so I have to drummel out the inside to fir the hop up assembly.  The valve I am using is the guarder low flow valve I think.  I saw the other post about this and you guys said that there is a gas leak or a torn bucking.  I dont think thats it because it has this low of an fps with both the EVO II bucking and the RA-tech bucking.  If you guys want I can post some pictures either tonight or when I get back from vacation.  also I get almost no hop up adjustment.  I put a new bucking into my original upper that I cut the barrel down to 9 inches and that one shoots 285-290 with .25s with the same bolt, so I know I can install bucking correctly.  I was think that with the same bolt and bucking but adding the RA-Tech 6.01 370mm barrel I would get around 330-345 with a .25 but it's just not happening.  

thanks for any help

Edited by Migwl300, 08 August 2014 - 03:17 PM.

#2 Chuck S

Chuck S


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Posted 09 August 2014 - 04:10 PM

Is this a LM4 or something else accidently in the LM4 area?

All ya need for 400fps is a high power gas valve in the bolt.  On the LM4.  A gas gun.

9 inch barrel?

-- Chuck

#3 Jatodude



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Posted 11 August 2014 - 12:39 AM

What I would try doing, is switching your inner barrels inside your uppers. Try the 9 inch inner, inside your Dytac barrel (is it the Dytac 16" Nighthawk barrel for the KSC?). If the 9" inner yields poor results inside your field setup, you know that the full length barrel isn't the problem. The problem here may be from the custom-fit outer barrel.

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