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Post by Da Vinci - Да Винчи on Jun 4, 2012 9:25:38 GMT -5
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Krutch
New Member
To all those against us, good luck.
Posts: 499
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Post by Krutch on Jun 4, 2012 12:29:36 GMT -5
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Post by Da Vinci - Да Винчи on Jun 4, 2012 13:06:39 GMT -5
Okay, let me rephrase, then.
I know that porting towards the front of the gearbox does affect the FPS, however the KWA's is at the very rear, and is said to be used on all of their AR variants, from their standard-length M4 variants to their M16 variants. Is it possible to change out the inner barrel to a longer barrel without changing cylinders?
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Post by TheEnd on Jun 4, 2012 23:25:50 GMT -5
The port in the cylinder is a way to adjust the fixed volume of the standard cylinder. A standard full non ported cylinder has enough air to drive a bb down a 509mm barrel. When the piston starts moving forward so does the bb, and when the piston stops, the bb has left the barrel.
Now, imagine you cut the barrel in half. The piston starts moving and so does the bb, except now the bb will leave the barrel before the piston has hit the cylinder head. This means the system is over volumed. The cylinder has too much air for the barrel. You fix this by porting the cylinder to essentially cut the cylinder in half. When you have a port the piston does not build compression until after it has passed the port. It is also possible to under volume the system by using a port which does not build enough compression.
Cylinder porting is not a good way to control FPS. If your rifle shoots hot you should adjust the spring, not the port. There are plenty of neat tricks you can do with volume balancing. It's also worth noting that TM cylinder numbers are not always spot on perfect. There is usually a barrel length range such as 363mm-400mm. If you want to learn how to exactly balance your system (it's worth it) guides are out there.
It is not uncommon for companies to install a single type of cylinder for all of their AR models. If I'm not mistaken G&P uses a solid cylinder in most of their AR's regardless of barrel length. JG has also been known to install a type 1 cylinder in some runs of their M16's. It saves money and makes it easier to assemble the AEG's if they all take the same box.
To answer your questions:
The reason KWA puts their port near the rear is because it supports a 363mm barrel which requires most of the cylinders volume.
Changing the inner barrel without changing the cylinder is possible. You can mess with the system with bore up kits, spring rates, and bb weights, but that's advanced stuff. You cannot run a 509mm barrel from a type 4 cylinder. As long as your barrel is within the cylinders range it should be OK.
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Post by X on Jun 5, 2012 11:32:48 GMT -5
The port in the cylinder is a way to adjust the fixed volume of the standard cylinder. A standard full non ported cylinder has enough air to drive a bb down a 509mm barrel. When the piston starts moving forward so does the bb, and when the piston stops, the bb has left the barrel. Now, imagine you cut the barrel in half. The piston starts moving and so does the bb, except now the bb will leave the barrel before the piston has hit the cylinder head. This means the system is over volumed. The cylinder has too much air for the barrel. You fix this by porting the cylinder to essentially cut the cylinder in half. When you have a port the piston does not build compression until after it has passed the port. It is also possible to under volume the system by using a port which does not build enough compression. Cylinder porting is not a good way to control FPS. If your rifle shoots hot you should adjust the spring, not the port. There are plenty of neat tricks you can do with volume balancing. It's also worth noting that TM cylinder numbers are not always spot on perfect. There is usually a barrel length range such as 363mm-400mm. If you want to learn how to exactly balance your system (it's worth it) guides are out there. It is not uncommon for companies to install a single type of cylinder for all of their AR models. If I'm not mistaken G&P uses a solid cylinder in most of their AR's regardless of barrel length. JG has also been known to install a type 1 cylinder in some runs of their M16's. It saves money and makes it easier to assemble the AEG's if they all take the same box. To answer your questions: The reason KWA puts their port near the rear is because it supports a 363mm barrel which requires most of the cylinders volume. Changing the inner barrel without changing the cylinder is possible. You can mess with the system with bore up kits, spring rates, and bb weights, but that's advanced stuff. You cannot run a 509mm barrel from a type 4 cylinder. As long as your barrel is within the cylinders range it should be OK. This is a very good post. |
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Post by Da Vinci - Да Винчи on Jun 5, 2012 12:04:22 GMT -5
TheEnd, that answers a lot of questions, but leaves one. How do you determine the cylinder's barrel "range"? The KM4 uses a 395mm stock barrel, and shoots at around 400 FPS, so what's the maximum barrel length I can change to keeping that same general FPS?
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Post by Tank on Jun 5, 2012 12:52:27 GMT -5
This is a very good post. Not only is it a good post, I think it actually explains something that I've seen crop up recently, when I changed my barrel out from the 395 to my 455. Occasionally I get a BB that doesn't shoot very far. It's seldom, but it does happen. I really didn't put two and two together, about the possible vaccum that I have created, until just now. |
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Post by TheEnd on Jun 5, 2012 15:06:04 GMT -5
Arnies has a quick guide with the basic barrel ranges that the cylinders will support. As I said, this is a general guide and is not perfect. Volume balancing is extremely complex and what works in one setup will not always be the same for other setups. If you want the right cylinder, do the math. Just changing from a 6.08 barrel to a 6.03 changes your balance, start messing with barrel length and diameter and you can really change things. The KM4 uses a 395mm stock barrel, and shoots at around 400 FPS, so what's the maximum barrel length I can change to keeping that same general FPS? Again, FPS is not something you should be aiming for while volume balancing. If you gain or lose FPS it's because your system was not properly balanced. According to the guide if you have a type 1 cylinder you should be OK up to a 460mm barrel. However 460 is pushing the limit and has the potential to be under volumed. AK's generally run a full cylinder with a 455mm barrel. Don't forget you can drill your own ports in a full cylinder or adjust pre existing ports.
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