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.
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?