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My FPS is staying the same?


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#1 kwaispro727

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Posted 25 March 2013 - 05:45 PM

I installed a 190mm 6.01 barrel.  The gun operates just fine.  Except... the FPS is exactly the same.  I'm stumped.  I was really hoping it would go up some, into the 450 area.  Is my compression bad?  What could be causing that?

#2 GenTarkin

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Posted 26 March 2013 - 07:08 AM

Are environmental conditions the same?

#3 kwaispro727

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Posted 26 March 2013 - 11:40 AM

Yes. I chronoed it in my basement, same temperature, same gas, KWA .20s.  I also tried bioval .20s and got the same results.

#4 Chuck S

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Posted 27 March 2013 - 02:45 AM

Your stock barrel might have been 6.01mm or very close to start with.

Is 190mm a typo, or is the 220mm barrel length listed in my MP7A1 manual a typo?

If these lengths are correct:  Muzzle velocity in gas guns typically varies with barrel length and if yours if over an inch shorter it would be slower.  The tighter barrel uses gas more efficiently and usually increases velocity.  These two can offset each other.

-- Chuck

#5 kwaispro727

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Posted 27 March 2013 - 05:12 AM

If the stock barrel was a 6.01, then there should definitely have been an increase in FPS with the lengthening of the barrel.  The stock length is 162mm I believe.  (I know it's in the 160s).  I posted about this in another thread already as it came up that another member installed the same barrel.

http://kwausa.com/fo...showtopic=10973

#6 Huntsman82

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Posted 27 March 2013 - 05:14 AM

I have that exact same barrel. It is a little longer than the factory barrel. Thats is strange that its listed as being shorter.

#7 GenTarkin

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Posted 27 March 2013 - 06:47 AM

out of curiosity, take a micrometer to your stock inner barrel, would be interesting to see its measurement.

#8 kwaispro727

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Posted 27 March 2013 - 07:26 PM

I don't have one :blinded:

#9 philsaudio

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Posted 28 March 2013 - 05:27 AM

They usually put these on sale for $9.99. If you are airsoft teching you really need one of these to communicate and put the right parts (shims) back in the right place .

http://www.harborfre...iper-47257.html


wage peace

#10 Outlaw1995

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Posted 28 March 2013 - 07:25 AM

Check your groupings on paper... I'd imagine your shot placement has a tighter grouping versus when they were shot with the stock barrel.

#11 kwaispro727

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Posted 28 March 2013 - 09:06 AM

I shot it yesterday, I didn't even bother with the paper I could see a difference when I was just making sure it was working.

#12 Chuck S

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Posted 28 March 2013 - 03:30 PM

I've no clue where the 220mm barrel length in the HK/Umarex/KWA manual came from but it's obviously wrong now that I have the gun on my desk!  It's roughly 205mm to the front edge of the flash suppressor.

So it ain't barrel length!

-- Chuck

#13 kwaispro727

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Posted 29 March 2013 - 01:52 PM

205mm is still much longer than it should be... Have you changed it?

#14 Chuck S

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Posted 30 March 2013 - 03:43 AM

Stock inner barrel should be 160mm.  No way the HK/Umarex manual could have been right about 220mm as the outer barrel to the tip of the flash suppressor is only about 205mm.  190mm inner barrel will project into the flash suppressor region and in itself should product a bit more muzzle velocity.

Still a puzzle why you're seeing a decrease.  Gotta be in the bucking/nozzle seal area I'd think.

-- Chuck

#15 kwaispro727

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Posted 30 March 2013 - 06:41 AM

Another member had the same issue with the same barrel.  I think what's happening is the 6.01 bore is too tight for the gas to expand enough but the added length counteracts that.  I believe the Edgi barrel once made for the MP7 were 6.03, which are basically as tight as you can go with the stock barrel before losing FPS.

#16 Chuck S

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Posted 30 March 2013 - 07:54 AM

Could be.  The friction of driving a BB down a very tight bore may be a factor that offsets the sealing effect of the tight bore barrel.  Muzzle loading black powder guns shooting round lead balls typically use a greased patch around the ball to help seal the undersized round ball in the barrel -- hardly possible in our guns.

-- Chuck




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