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Cylinder to Barrel Volume Ratio


renegadecow

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I know it's important to have sufficient cylinder volume for longer barrels, but I struggle to find hard data on the effects of the opposite: too much cylinder volume for shorter barrels. All I keep reading is "wasted air" or simply "not optimized" but just how much loss are we talking about? And for the record, it's to do with my XP-100 (custom BAR10) which is running a 247mm inner barrel on stock cylinder volume. The stock cylinder is supposedly ideal for a 500mm or so inner barrel which puts me at about 2x the volume I need. In comparison, that's like running a type 0 AEG cylinder on an MP5 barrel, which I haven't exactly tried in the past so I have no idea what sort of fps gain is at stake. I'm wanting an extra 20-30fps and was thinking of buying a new spring for it, but if simply correcting my cylinder volume will give me that much gain then it'll save me the few bucks plus not have to make the bolt pull any heavier.

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I don't think you loose FPS withe a wrong ration like you would with a long barrel and ported cylinder, but I think it's more to do with the excess air causing turbulence to the BB after its left the barrel and decreasing accuracy. A wide bore barrel would likely cure this problem I think.

 

These are my own theories so not sure if they're right

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I used to think the excess air = turbulence was the only problem but I've been reading about how the bb leaves the barrel before the piston comes forward enough to achieve peak pressure. The principle behind a ported cylinders other than matching a short barrels volume is that the piston is given extra time to accelerate getting that much desired peak pressure for optimal fps. But again, I haven't read any numbers on this. I'd experiment on it myself but I'm still waiting for my new chrono in the mail.

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Another reason for 'balancing' barrel and cylinder volumes is that you get a quieter gun out of it. All the G&P stubby M4s that are also loud all have full cylinders but shoot around 280-300...put an M16 front end on and suddenly they're doing 360.

 

The stock VSR/BAR cylinder has way more volume in it for a even the 550mm barrels if I recall correctly - I would look at measuring the dimensions of the area that the piston o-ring seals against to the cylinder head and apply the maths for working out the volume of the cylinder. Then do the same for the barrel (you may have already done so). You should then be able to see how many barrels worth of air are in your cylinder - I think the ideal is about 1.5-1.7 from what I've read over the years.

 

As for a 20-30fps boost, try adding some shims onto the spring for precompression while avoid coil binding? A bearing spring guide in an AEG generally adds about 15fps, the lug from inside a TM piston head adds 25  (or drops if you dremel it down into a plate) from experience.

 

A point of interest would be that perhaps reducing cylinder volume to a less overvolumed state improves FPS but this would be a bit of a faff.

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It seems I've found the answer elsewhere, casting my net into various forums and digging up farther back into the past.

 

Ultimately there IS a definite loss in fps in using an over ratio of cylinder to barrel volume. It's not so much that the system becomes inefficient but that there is gain in allowing the piston to accelerate a little before the system is sealed and pressure starts to build, but the difference is not by very much, at least not as much as I need in my gun (under 20fps from what I've read).

However, one important detail I came across is that the ratios change with respect to ammo weight and more so with shorter barreled guns. The heavier the bbs the more air volume is required to generate sufficient pressure in optimizing the system. While I've found hard data, the numbers and even the ratios vary from person to person which I'm guessing is from user error.

Now, the problem is that I don't actually have one. My gun is working perfectly. The bb seems to exit the barrel just a hair of a fraction before the piston is fully compressed (piston never audibly slaps against the cylinder head), but that's from using .30g bbs primarily. As a test I tried using .20g bbs and the difference was drastic (will confirm when I get my chrono) as the bb noticeably leaves the barrel too early, almost at just the first half of piston movement and then the piston is free to almost dry fire the rest of the way. From what I read, in example, the bb weight used can make as  much as a 2J jump when using an MP5K barrel on a type 0 cylinder. As it appears my cylinder volume is just right despite well being over 2x the norm, using .30g bbs making up for it and I'm guessing is more optimal with .40g (will try testing this too). For the fps boost I required, I'll likely look up into RSMs suggestion of putting spacers behind the spring if not change the spring a next step higher. The stock JG BAR10 spring is rated between an M110 and M120 so I'm thinking of getting an Element M125 or a King Arms M130. And since springs aren't all that expensive I may as well try and document alleged inconsistencies on non-linear springs.

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