Same Barrel Assembly, Different FPS readings?
 

Right now I'm doing some work on a friends VFC 416 CQB.

I've also been doing some testing with one of my own guns (originally a VR16 Fighter CQB) to create alternate inner-barrel setups as a way to quickly change my FPS.

Both guns came, with what I assume, is the same stock inner barrel and hop-up assembly; ~276mm brass barrel, rotary hop-up.

Here's a picture of both barrels for reference, mine is the one on the bottom. The top one is completely stock: no modifications, and has never been opened from the factory. The only difference is that the one on the bottom is flat-hop'd. I previously modified the stock bucking, and replaced the default nub with a Prometheus flat tensioner.

http://i.imgur.com/pyoClUZ.jpg

The issue I am having is that between the two barrels I get a difference of about 10fps when loaded into my VR16. The stock barrel gives me 375-380 fps, and the flat-hopped barrel gives me 365-370; hop-up turned off (Xcortech x3200). For the life of me this doesn't make any sense, unless I seriously messed up my flat-hop job and created an air leak in the flat-hopped barrel? One slight difference I noticed is that when I look down both barrels, the nub of the stock barrel protrudes into the barrel ever so slightly.

Does anyone know what might account for this FPS difference?

I think you answered your question, you have a leak probably around the bucking. Try sealing it to the inner barrel with some dental floss or teflon tape.

Increased contact area from the flat-hop setup.
Possibly flubbing of install

Maybe a small air leak, though 10-ish FPS is not extreme. I've seen two side by side AEGs act very differently despite being exactly the same. Do you get a good consistent shot out of both? But definitely check your seal around you barrel.

Did you rotate the rubber and remove the alignment channel

Yes, and yes.


I was considering the dental floss mod, but I find that the stock rubber is nice and tight in the hop-up. I usually lube it on the outside with a small amount of silicon grease, prior to inserting it, but even with lube it's tight to get in.

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Check them both with the hop set for .28s or whatever

Did you fill the alignment channel?

Noooo? I didn't know this was a thing. What do you fill it with?

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The original tab of the rubber.
I dont even rotate the rubber when i flathop it

My understanding of how to flat hop is you remove both the alignment tab and the nub of the bucking, then rotate 90deg to a clean portion of the rubber.



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Something. Silicone, rtv, sticky tack... you may loose air from there depending on how well the rubber seals the end of the barrel

I shave the nub so smooth that its just as smooth and flat as the rest of the rubber

I filled in the alignment channel on my flat-hopped barrel with RTV silicone, but only had a noticeable increase of about 2 or 3 FPS, compared to the barrel before the filling.

The only other thing I can think of that might be affecting the FPS, is what Steve suggested; the area of contact. I'm beginning to wonder if there might be some manufacturing tolerance variations in the diameter of the two barrels which would account for the difference? Either way, I'm happy with the result as the FPS appears to be a bit more consistent overall.

I also filled in the alignment channel on a 300mm madbull tightbore I have, and got a bump of about 10-15fps. I'm going to chrono it a few more times with different weight ammo, as I feel the results are too good to be true.