Hi guys, on the LeHui Kriss Vector V2, the barrel is quite short, but still gets about 230 fps out of it…if I put a non-ported cylinder in and a longer barrel (maybe like 33-35 inches), how much fps change would there be?
30" is a tad too long I think if my math is mathing right…
(30" ≈ 760mm)
With just a cylinder swap and without a spring change the FPS would probably be about the same, slight increase but horrendous VE without a longer barrel. But you’ve already figured that out. 
Not that much in the way of upgrade parts for Vectors. O ring, spring, cylinder… pretty proprietry after that. If you upgrade the piston make sure it’s Vector compatible ( angled on the end ) or it will lock up first cycle.
Personally, I wouldn’t change out the ported cylinder unless you’re planning on running a permanent suppressor. If anything I’d probably port the cylinder more and just give it a mild spring upgrade, an alloy barrel, maybe a motor upgrade. 
One of my favourite gearboxes… Le Hui put the yards in with that one… cool design.
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I’d recommend the maximum length for an inner barrel to be 380-400mm. Anything longer and the volume just isn’t there, you get very inconsistent shot to shot fps. Gets worse the longer the barrel gets.
80% cylinder and 275mm-ish barrel is a good combination in them. Then just use the suppressor.
Worth noting, vector barrel is 10mm outer diameter, most barrels are 9.5mm. a couple of wraps of aluminium tape the glue the barrel in works if you cant source a 10mm od barrel.
The standard outer barrel is cw threads. So normal barrel extensions won’t work. Kind of limits you to the stock suppressor for thread on option, or you can use the push on flash hider barrel with a 19mm but the grub screws to hold them on just ream into the nylon barrel.
There once was alloy outer barrels available with 14mm ccw threads, but I haven’t seen those in years at this point.
As @Friendly_Fire stated, just using a different piston will cause it to lock up.
You can use any piston that has a solid body, and cut/sand the rear corner down to match the stock piston, but definitely check it cycles with the spring retainer installed manually before pulling the trigger.