Complicated thinking: Why match cylinder porting and barrel length?

[Cushak]

I was reading through this, and it got me thinking:

Would the way to find the perfect cylinder length (or even where to cut holes in a full cylinder to get a more efficient go) to do a simple volume analysis?

That is, you'd want the same volume (not mass - it's constant no matter the pressure) both right before compression starts and the bb is still at the start of the barrel, and when the piston has reached the end and the bb is just about to leave the barrel?


That said, Mcguyver, on Semi I believe that you would still get some vacuum suck back if your cylinder wasn't matched with your barrel. Because as the BB travels down the barrel, the mass of air inbetween it and the piston head does not change. (assuming the piston is momentarily at rest) The volume however is changing, resulting in a forced expansion of air beyond a pressure of one atmosphere, creating a back pressure. (again, only if your cylinder and barrel length aren't matched (cylinder too short for your barrel length)).

I could be wrong, as I'm not fully learned in the whole cycle the AEG goes through, and the timing of when everything happens.

Using a tight-bore would mean you could get away with a smaller cylinder, correct? (technically speaking, though the effects may be negligable)


Basically what i'm saying is this: Say you have a 450mm 6.02 barrel (assuming the bb is waiting for the compression right at beggining of the barrel). That means that your barrel has 12.8 cubic cm's of volume, so, right before the piston head begins compression, you'd want 12.8 cubic cm's of volume between the bb and your piston head.

Am I right or wrong in my thinking?

[ILLusion]

In a perfect world... in a perfect system, your theory is 100% correct.

But we don't live in a perfect world, and the AEG system isn't perfect. The air seals within the compression chamber are not 100% efficient, so all of the following areas of air loss must either be corrected to get your formula to work, OR you must allow a greater volume of air to compensate for the lost air:

1) piston head to cylinder wall
2) cylinder wall to cylinder head
3) cylinder head to air nozzle
4) air nozzle to hop up rubber
5) air nozzle to BB
6) BB to inner barrel wall

Variances in temperature can also affect the demanded and supplied volumes of air due to expansion of materials, as well as the quality of compression (stiffening of o-ring rubbers in cold, etc)

Easiest way to go about all this? Just get the pre-manufactured cylinder sets out there. Less headaches, and would work out to a better finished product... and probably even cheaper than any of us could produce. All the R&D is already done... why re-invent the wheel?

[Cushak]

Originally Posted by ILLusion

All the R&D is already done... why re-invent the wheel?

Because I'm bored and distractions are welcome. Even if they're difficult. It's what I do. Lol, in all seriousness though, I was never thinking I'd use that to make myself a cylinder or anything, I'm not that cheap. Plus, in first year engineering, everything is a perfect system:mrgreen:.

[Killbucket]

If you get bored with all this stuff, try designing an audio speaker box. Lots of math to play with there, but the bottom line is if it doesn't leak, it will work satisfactorily.

I have an M4 that I ported the living crap out of before I even knew why this was done. I figured it let the cyl fill faster and reduced "suck-back"...
I just measured a CA cyl at the shop and matched dimensions, adding a few more holes further back, tapering in size. So there are three holes in a row, largest one at the start of the power stroke. There are four of the hole groups going around the cyl. The porting effect is as amplified as it gets in this gun. This gun has a high-speed Systema motor, 170 spring, metal bearing spring guide, metal bushings, reinforced piston/head, and a Stainless steel cushioned cyl head.
How does this gun work? Terrible on semi. The bb will go about two feet before dropping. On full auto, yeehaw! It will spit a solid stream of hard-hitting pellets with very acceptable accuracy for CQB.
I was building a short gun anyway, but I have since learned that putting a long barrel with this gearbox results in a dog of a gun with lousy range.

[Kimbo]

Holy Frankengun! Don't think I've ever seen a gun as customized as that.

[Pug_674]

When you say "unported cylinder" are you refering to the fact that the cylinder isn't made / modified to the specifications of that specific model of gun?

[Greylocks]

It means the cylinder has no ports in it at all. It's a straight tube instead of one with an oval opening near one end.

[Mr. G36!]

Sorry if this is a stupid question, but when you swap out your cylinder to allow for the installation of a longer inner-barrel, do you have to replace the cylinder with the original one if you were to replace the barrel with the shorter one, or will the new cylinder work properly with any length up to what it's designed for?

[ILLusion]

If you're using a short inner barrel with a cylinder meant for a longer inner barrel, you subject your gearbox to more stress (projectile has long since left the barrel, and thus, there is no longer any backpressure on the piston head to reduce impact on the gearbox), ROF is slowed down, and velocities are typically not as high nor as consistent.

[freewheelin4eva]

so I have been thinking. If a barrel of a particular length only takes so much volume of air to work properly, would a Cylinder bore up kit then have a negative effect on performance. Or has changing the displacement been thought of, and the position of the port on cylinder been moved accordingly?

[Styrak]

Like Ilusion just posted, if the cylinder is too big for the barrel, the piston will have no BB left to push out the barrel (it will have already left) and the piston will slam into the cylinder head.

[freewheelin4eva]

well yes I understand that, maybe I just did not ask my question right. If I have say an M4 style gun, w/t the proper cylinder, what is the point of getting a cylinder bore up kit. since all that will do is create more air displacement, which really is not needed. and will then create the piston having no resistance at the end of its stroke.
I guess you could solve this by shortening the stroke of the piston. but that is probably not worth it.
so bore up kits I guess are they a worthwhile investment, or just a waist of money.

[Styrak]

They are only a good investment if you get an extended barrel.
For example, if you put a suppressor on that M4, and put in an extended barrel that extends to the end (or just before the end somewhere) of the suppressor.

[ILLusion]

Bore Up kits are generally used for longer than normal barrel lengths, or for high velocity setups.

Pneumatic efficiencies are much more glaringly apparent above 400fps with exponentially higher air losses when shooting with an M120 or higher. Thus, bore up kits are sometimes suggested as a way to reclaim lost air volume passing through the inner barrel at these higher velocities.

As a side note, bore-up kits should only be used with M120+ springs - I'd recommend you begin considering it once you step in to M130 territory. Springs weaker than that may not have the strength to expand the o-ring enough during the compression phase.

[Hectic]

Sorry I think this was answered earlier in the post in like 2005 but I just want to make sure that the cylinder in my JG M4A1 is the same as the one in my TM MP5 SD/6 so I could put the M4 barrel in the MP5 and vise verse without any adverse effects.
Then I could put M4 barrel in Mp5sd with a direct silencer attachment an outer barrel extension and a flash hider and make a I don't know what to call it a custom MP5 carbine frankengun thing.