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Cylinder Porting & Inner Barrel Lengths
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TOPIC: Cylinder Porting & Inner Barrel Lengths

Cylinder Porting & Inner Barrel Lengths 5 years, 6 months ago #1

  • Napster
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Credit for original chart goes to ArniesAirsoft in the UK, but I'm gonna suggest some alterations. First, here's the chart and a list of common models that carry the said lengths of barrels.






My changes:



No hole cylinder (Type O)

Only 450 - 550mm barrels. Do not use longer than a 550mm barrel in an AEG (I can tell you from experience, ask anyone about my SR-12 with a 650mm barrel), unless it's the extended cylinder/piston such as an SR-25, then you can go up to 650mm with a no hole cylinder. Very few AEGs can use over 550mm barrels due to cylinder air volume, but they are made because some bolt action sniper riffles have modified hop up chambers available that allow the use of AEG barrels, and their cylinders are bigger than an AEG's. On the other hand the extended cylinder/piston models have fewer choices for pistons and gears on the market.


4/5 hole cylinder (Type A / 1)

350 - 450mm. You can use this with an M4 length barrel (363) despite what the chart suggests, especially if tightbore and even more so if you use a sorbo.


3/4 hole cylinder (Type B / 2)

171 - 362mm - If your gun has shorter than an M4 length barrel in most cases this is the cylinder for you.


1/2 hole cylinder (Type C / 3)

Under 170mm barrels. These are very very short riffles and sub-machine guns for CQB such as the MP5k, some M4 "stubby" variants, some CQB variant G36s, scorpion, uzzi, MP7, MP9, etc. For reference a pistol barrel is 80-110mm long depending on model.


If you use a sorbo and you are near the upper end of the barrel length range, you may want to step up one level in cylinder.


What happens when you use a cylinder with higher air volume than what the inner barrel needs? I think Travis (T5000) from SAP put it best. Your're still pushing out air after the BB's gone, it's a waste and you lose FPS.


What happens when you use a barrel longer than what the cylinder volume can support? Well from my experience on the SR-12, it threw my hop up way off and affected the spin access causing hooks to one side or the other and even with overhop I was getting at most maybe 150' distance shooting 520fps, when I should've been getting 250-300+ with a 6.01, the air seal I have, the modified hop up, and the quality Madbull .36 and .43g BBs I was using. Some were getting flung towards the sky, others were dropping at 100' due to the inconsistency caused by the vacuum in the barrel.





Now, what type of cylinder to get? Well if given the choice I recommend stainless steel over aluminum, and aluminum over brass, but more important than the material is the finish both outside and in. Stay away from coatings in the cylinder, you want it to be polished or polish it yourself but no coatings. On the outside, I recommend a smooth surface, not that ribbed stuff, sometimes the tappet plate likes to hook on it.
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Last Edit: 5 years, 4 months ago by Napster.
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Re: Cylinder Porting & Inner Barrel Lengths 5 years, 5 months ago #2

  • SLINKEY
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Well why wouldn't you just do the math and get it exactly right.. ontop of what travis said, having too much air volume behind the bbcauses turbulence around the bb as it exits the barrel which makes your bb's unstable in the air.

If your looking for raw accuracy do the math and get it perfect. Use a full cylinder.

Find the volume of your barrel. Than match that volume to the demenshions of your cylinder. And drill a hole or make a fancy hole it doesnt matter. At the length that closest matches the volume of air inside your barrel.

Iv personally seen this make more of a difference in accuracy than a tight bore barrel

Re: Cylinder Porting & Inner Barrel Lengths 5 years, 5 months ago #3

  • Napster
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Andyyyyyyy you got Khang and I going crazy in the private forum, because I'm trying to match the barrel lengths to cylinder lengths and getting numbers that don't make sense, then Khang's found a correlation measuring from the back but I think that's just a coincidence for that specific length because it would make no sense as going down in barrel length or going up would change the volume in the wrong direction by using that.

Then Khang got out his calipers for more precise measurements and we're still not getting the numbers expected, because we both can't find anything wrong with my equation, the only possible fault was a bad measurement of cylinder radius, as I used a simple ruler, but that wasn't the case as we figured exact dimensions once Khang got his calipers, and even a mm up or down on the diameter, the volume doesn't change very much.


Using the basic cylindrical shape volume formula of area of circle times the height, V = pi * r^2 * h. I'm going crazy here, because according to those calculations I would need only about 23mm of the cylinder (from the cylinder head) for a 363mm 6.03 barrel, and the rest can be ported. But that's like the 1/2 or even a 1/4 hole porting.


I can only assume that there HAS to be a ratio of cylinder volume to barrel volume that's not 1:1. Maybe 2:1, cylinder to barrel volume, probably to maintain a certain pressure in the barrel. Now my head hurts and I'm guessing on that 2:1 but I'm getting just over half of the length of cylinder I had expected to match the barrel, help us out here, because by 1:1 ratio, then a full cylinder would be able to support a much much longer barrel like say 1 meter.


The other thing is that since they do 1/4" increments on the porting, well that leaves a lot of barrels that don't have exact volume matches, so they just use the next bigger volume.
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Last Edit: 5 years, 5 months ago by Napster.

Re: Cylinder Porting & Inner Barrel Lengths 5 years, 5 months ago #4

  • k3n
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Yah... my engineering buddy and I kicked some butt...

Click Here to Download Calc Beta (has the equations)

Ignore the left part of the excel sheet and focus on the right portion.

The link denotes the ratio, the determinants of the ratio and how we can apply the ratio to the calculations of a cylinder. The results are almost spot on (variance of literally 1.5mm). The ratio was determined by following KWA spec guns (since I could physically measure the KWA barrel and cylinder). We determined 1.857753 was the ratio between the barrel volume and the cylinder volume. We assume that the cylinder volume is higher to account for extraneous variables and the need to increase air pressure to 400 FPS with the spring. We then used the ratio to calculate the new cylinder volume (barrel volume x ratio). From here we are able to find the new cylinder dimensions and how much to cut by subtracting the overall barrel length and the new effective length.

Not exactly what we wanted but at least we can calculate the approximate cut needed. We are pretty sure the ratio is at least a good estimate to go and find approprate cylinder lengths and such but we are also pretty sure that the ratio we have is not exactly correct. We are also absolutely sure that this is not the original method used to find cylinder length but we can be sure that this is a good way to estimate length haha
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Last Edit: 5 years, 5 months ago by k3n.
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Re: Cylinder Porting & Inner Barrel Lengths 5 years, 5 months ago #5

  • Napster
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I knew it had to be a ratio closer to 2:1 than 1:1 to match closer to where the ports are made for the recommended lengths. 1.86 sounds about right man, good job. I assume then you used the example of a ported KWA cylinder and measured both volumes to come up with that ratio. It sounds legit to me and it sounds in the range I was expecting. So now we can multiply the barrel volume by ~1.86 to get the required cylinder volume, which is much closer to the expected number. I think that's a great number to use Khang as it gives relatively expected numbers. Otherwise I would've just used a 2:1 ratio and have a little extra air in the cylinder.



Now as always, it wouldn't be a good project if I didn't uphold the tradition of throwing a wrench in there to have us all scratching our heads, so here it is.

One of the things that will determine the pressures is how fast the piston travels through and how long it takes for that cylinder air to transfer into the barrel, in other words what would equate to the fps of the gun, which means that a 300 fps gun will have a different ratio of cylinder to barrel volume than a 500fps gun. Although I think what you have there is a good average for the majority of the 370-400fps guns that we'll have on the field shooting full auto.
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Re: Cylinder Porting & Inner Barrel Lengths 5 years, 5 months ago #6

  • SLINKEY
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Sorry i wasnt too clear. It been a long time sence iv worked on aeg's
more hpa, and daytona guy now.
Your correct, it is a higher ratio over the barrel volume. Didnt nean to cause unrest just forgot that haha

Its about time actually know what the heck im talkin about haha

With a heavier spring it will cause the compression "push" to be reduced a ton. The push i mean is how long it takes to volumize the pison head ports and expand the phead oring. Some ported guns with heavy springs can actually have so much pressure the port does nothing at all. Meaning port sizes come into play
This is where systema came out with the tear drop port shapes. Ugggghbcrap..

There variables are the reason i gave up on aegs as a fine tuned dmr, or sniper. Theres too many variables that will change everything shot to shot. even with a variance of +/-0 fps you can still get over or under volumization.
Another huge variable is the crown on your barrel. A long shallow slant crown from what i have experienced can make a huge difference in accuracy also. If the crown is done right the whole turbulence factor can be tossed out the window.

My answer to this dielemma was a polarstar and a calculator haha
Last Edit: 5 years, 5 months ago by SLINKEY.
The following user(s) said Thank You: Napster, k3n, Flash
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