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FPS and Barrel Length


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#1 SuperFly

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Posted 05 September 2012 - 09:52 PM

I wanted to start a discussion after I had some puzzling results tonight. I have two lm4's, one I bought used with a ra-tech 6.01 standard length tight bore barrel, guarder steel hopup and bucking. When I first got the used one it was shooting around 380 fps. I took the whole thing down to individual pieces, cleaned everything with alcohol and then did the frog lube seasoning process twice, on everything. After everything was wiped down and it was all put back together, I lubed the friction points with some frog lube and did a chrono test. I was surprised to find the fps had jumped up to 450fps.

I had some parts come in today, another tightbore (this time a 6.02 500mm barrel), another guarder steel hopup and an assortment of hopup buckings for the guarder chamber. I assembled the two barrels with the chambers and new buckings (same brand), and then using the same lower with different uppers but same bolt, I tested fps again.

The 500mm 6.02 barrel is barely coming in above 360fps, while the standard length 6.01 barrel is chronoing over the 470 mark. Is that .01mm difference causing the fps variance? Seems like a small enough difference not to matter. I was expecting the 500mm barrel to have higher fps, and I would not be as puzzled if they were even close to the same fps, but the +100fps difference has me scratching my head.

The only real difference (besides barrel length) is the fact that the upper with the 500mm barrel has not been cleaned, seasoned and lubed with frog lube.

Any ideas?

Edited by SuperFly, 05 September 2012 - 09:53 PM.


#2 akiraspeedstar

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Posted 07 September 2012 - 08:08 AM

While I use Frog Lube in my LM4, I highly doubt it gave you a 70fps increase unless before you cleaned and lubricated the gun it was gunked up or binding some where. Also I don't believe the .01mm would reduce the FPS by a noticeable difference. Also with the increased barrel length, especially with a GBB, I'd expect a significant increase.

What chrono are you using?

#3 SR105

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Posted 07 September 2012 - 08:56 AM

Probably where you're having issues is the bolt. Since you have two what you can do is swap the 'high' fps bolt into the other rifle and see what results you get. Also, to reduce the variables, you should use the same magazine throughout your tests. If you get the same results even though you swapped bolts then my next guess would be the hop up, which I assume you can check since you've disassembled and reassembled the hop up unit successfully several times already. Keep us posted!

#4 Blindside

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Posted 07 September 2012 - 09:49 AM

you also went up in barrel length, going to long in barrel length can decrease your fps. i know that that's how it is in aegs, i don't really know about GBBRs i'm still new to them.

#5 akiraspeedstar

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Posted 07 September 2012 - 10:24 AM

 Blindside, on 07 September 2012 - 09:49 AM, said:

you also went up in barrel length, going to long in barrel length can decrease your fps. i know that that's how it is in aegs, i don't really know about GBBRs i'm still new to them.

It is a possibility, like a AEG with its cylinder size affecting the volume of air, GBBs have their release valves which release an amount of gas. This would be affected by the valve port size and also the timing of the other components like rocket valve and valve spring. So it could be possible that the barrel is too long for the current setup.

Though that 70fps increase with just cleaning still has my eyebrow raised.

#6 SuperFly

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Posted 07 September 2012 - 11:36 AM

I was using the same bolt and mag, so that rules things out. However, perhaps the barrel length is too long. Thats the only thing I can think of. I was planning on having it cut down to fit in my mk12 barrel length, so I guess I'll go ahead and do that and then see where I am at.

And yeah, I was surprised with the fps jump after cleaning and frog lubing too.  I heard it's good stuff, but cmon! That seems odd. Maybe there was something wrong in the bolt that got fixed inadvertently.

#7 akiraspeedstar

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Posted 07 September 2012 - 12:20 PM

What chronograph are you using?

#8 SuperFly

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Posted 07 September 2012 - 01:38 PM

its an f1 chrony

#9 Cariza

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Posted 14 October 2013 - 12:15 PM

Bringing this subject back to life, Im considering going with a longer (500mm) inner barrel to hopefully add more precision. Falcon has one for our LM4.  Have you confirmed that the longer inner barrel really affected the FPS so much?  Currently with the EvoII hop up, maple leaf bucking and KM Head inner 6.02 barrel and lightly shaved rocket valve Im shooting 370fps with 0.40's.  I would not mind trading fps for precision as long as it isn't more than 30-40 fps.  My field doesn't have fps limits but it does have engagement distance limits.

#10 SuperFly

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Posted 16 October 2013 - 03:49 PM

As far as I know there is not a longer inner barrel with lm4 specific cut available yet.

Edited by SuperFly, 16 October 2013 - 03:49 PM.


#11 akiraspeedstar

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Posted 16 October 2013 - 06:04 PM

Ehobbyasia has the LM4 cut barrels that are 370mm and 500mm in length. Unlike the ASGI one where they brought in the KSC model the one shown on ehobby is the new cut.

#12 Cariza

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Posted 17 October 2013 - 09:29 AM

Yeap, I saw them there at Ehobbie though can't seem to find them anymore.

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