Cylinder to barrel volume.

[Blitzx]

Hi everyone. Ive been trying to find an answer to my question and i cant. Why do people say overvolume is bad but when you look at this tokyo marui use non ported cylinders on small barrel . Or i disassembled an ares ump45 which had a 205mm barrel but the gun still run a full cyllinder(non ported) so the gun is overvoluming for sure. But still they created and release the gun this way. So why would it be bad.

[Magwell]

The thought process behind it is that barrel volume should be proportional to cylinder volume if I remember right. The reason for this is that, in theory, the piston is not at it's forward/resting position by the time the bb leaves the barrel and is still moving slower than it could be with ports.

There's two things that can happen here from the negative perspective; 1. damage to gearbox parts over time. If the piston travels 50% of the cylinder with compressed air moving the bb, then the bb exits the barrel, it's like dry firing for the last 50%. 2. Joule creep with heavier ammo.

I know guys that live by the ported cylinder dynamic and guys that use only non-ported. For a 200mm barrel though, personally I would go ported for gearbox longevity.

[wind_comm]

overvoluming can be desirable if you're exclusively shooting heavier ammo, as you'd lose less energy at extreme range. at least that's the idea.

[Blitzx]

yeah im asking this because i got a mp5 with a 229mm barrel and im looking to increase to a 285mm since i can. my g&g already has a full cylinder i think because of the blowback system which need extra air to make the system works. i wish to remove the blow back system and wish to keep that full cylinder as i noticed on a video of tokyo marui. his gun shoots like a laser but still he got the original cylinder which is non ported. and got a 285mm barrel. so im confused because people keeps saying its bad. if it helps im using 0.32g bbs.

[ThunderCactus]

First and foremost; just because airsoft manufacturers build guns, it DOESN'T guarantee that they know what they're doing. If they built perfect guns, there would be no aftermarket mods or parts. And even if they did design a gun with all the mods and parts, they would still suffer losses from manufacturing.

Cylinder porting is a tradeoff of volume for pressure.
The lower the volume, the higher the pressure. This is how an M110 spring can shoot 380fps on a 420mm or 180mm barrel.

There are points of diminishing returns in multiple ways: (assuming M110 spring, 1.35j which would be 382fps on a .20)
If you are undervolumed, you may be shooting 1.35j with a .20, but find you shoot 1.2j on a .32
If you are overvolumed, you may be shooting 1.2j with a .20, and 1.35j with a .32
If you are grossly overvolumed, you may be shooting 1j with .20s and 1.2j with .32, but might also be at 1.35j with .43s
There's a balance point where you can be pretty close across the board. For example, my M14 is within <.05j across a multitude of BB weights.

It's a balancing act.
So manufacturers generally state with their cylinders "for 300-400mm barrels" or "for 200-300mm barrels". You want to be on the top end, so if you have a 229mm barrel, you'll want a 200-300mm port.
If you have a 285mm barrel, you'll want the 300-400mm port.
There are no real standards in airsoft manufacturing, so the same cylinder port could have different ratings depending on who makes it. Although it seems they tend to be rated by .20s, which is why you want to be near the top end; overvolumed a bit.

Now, all that being said, there's no reason for you to go from a 229mm to a 285mm barrel in the first place. You will not gain any accuracy, and you will gain exactly 56mm of range.
In airsoft, the sole advantage to having a longer inner barrel is making it easier to achieve higher fps, and a 205mm barrel can do 3.2j with the right build.
So just upgrade to a higher quality 229mm, and use a 200-300mm port, like a prometheus type D

If you have any other questions on how stuff works, it's probably already covered in this:
http://airsoftcanada.com/showthread.php?t=166583