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Cylinder to barrel ratio on a Tanio Koba barrel


Isamu

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I have an AK105 where I threw some hop up upgrades I had lying around, the gun shoots pretty nicely  actually, but I have been wondering which would be the most correct cylinder to barrel ratio for it.

 

At the moment the gun is like this internally:

 

* Retro Arms hop up chamber

* Tanio Koba 300mm twist barrel

* Prometheus purple bucking

 

* Gearbox is its own E&L Gen2 stock gearbox currently shooting at 322FPS with .20gs with a one piece full cylinder

 

Not exactly sure which cylinder a 300mm barrel should use (i'm technically handicaped ^_^U ) but given that TK barrels have those twists, Which should be the correct cylinder?

 

Thanks

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A rough n ready way but quite accurate I feel......

 

Divide barrel by 10 (tricky stuff I know)

In your case 300/10 = 30mm

 

Then add 15mm - why coz cylinder head protrudes aprox 8.5mm into the cylinder when fitted

then add a few mm for correcting AoE - so say 11.5 - 12mm.

Then add a few extra mm's for compression to build past port = 15mm (trust me)

 

So you end up with figure of 30+15 = 45mm

 

measure from the front of cylinder and 45mm is where the tip of the port should be

So you have 45mm of full unported cylinder

 

Feel free to do the maths and stuff but this does kind of correlate correctly

 

Or measure the port tip to front of cylinder eg: 52mm

subtract 15mm = 37mm, multiply by 10 = 370mm

The 52mm ported cylinder is about correct for a 370mm barrel

 

This is roughly going along the lines of a volume ratio of 1.75 or 1.8

If using much heaver bb's like say 0.30's or higher then add more than 15mm (say 20mm instead for DMR/Snipers)

 

I feel it roughly works out for most guns perhaps not quite perfectly for MP5k's but often they are overvolumed

(having 20mm or a bit more added to 10th of barrel length)

 

So answer to your question - the port's tip should be 45mm or just over to front of cylinder for a 300mm barrel

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Oooooh... nice stuff Lord*fruitcage*em, will try to size it at 42 to 45mm to give a bit more air for the twist and BB, although I use 0.28gs for rifleman duty.

 

Do you think I should ventilate the original one piece cylinder+cylinderhead or just buy a 363mm barrel cylinder+cylinder head and grind it?

 

Thanks 

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As you are using .28's then make sure it is 45mm or just over

Seriously it is a 10th + 15mm = 45mm on a 300mm barrel

So somewhere between 45 to 50 is fine

Don't worry about over volume on medium/longer barrels so much if using .28's

(you have to add on a fair bit to the stroke to push vol ratio over the guidelines)

 

As a rough guide you are looking for a 3/4 or 4/5 type cylinder but these references are rough to say the least

Also depends on the length of the port hole - some ports are say 15mm length - others are 20mm or more

The port length or shape doesn't matter - it is where the port finishes and the compression stroke actually starts

(just with larger length ports the reference to 1/2, 2/3, 3/4 etc... might seem to vary in real terms of measurement

 

You are using a 300mm on say .28's so you could use a cylinder designed up to say a 363mm limit and it would likely be about right

Bore up cylinders & TBB barrels hardly alter the volume maths by much - usually about 1% each

so really bore up cylinder & TBB will throw 2% into the mix of say a 1:1.80 volume ratio = 1.836 which is little to worry about

 

The rough n ready check is really just that

You got a barrel or cylinder and just need a general idea of what you can pair up with this or that

(without reaching for calculator or going off to a volume/ratio site trying to work stroke,  ID (aprox 23.8mm) of cylinder etc....

 

If you got a M4 carbine 363mm 3/4 cylinder to hand then give it a quick measure

it would daft to cut/port a full cylinder if you got something near it already

you just know you will need a full cylinder next month for a full size AK47 build or something

(I mean if you plenty of full cylinders in your spares box & nothing near the required port then yeah go for it)

 

It is just a quick guide for port position

 

The other one is a quick gear ratio finder....

Mark bevel gear with a pencil at 12 o'clock say

see how many bevel rotations it takes to turn the sector one full complete turn

Multiply the number of turns by 3 = the pretty accurate gear ratio of that gear set

 

6.25 turns x 3 = 18.75:1 - the actual ratio of most std 18:1 gears

5.75 turns x 3 = 17.25:1 - theactual ratio of many so called 16:1 gears - though a few are 5.5 or 16.5:1

4.25 turns x 3 = 12.75:1 - 12:1 etc.....

 

You get the idea unless you like counting teeth or measuring ports/stroke/diameters etc.....

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Can I butt in and ruin these nicely written, correct posts and say that it doesn't matter as much? With an m4 sized cylinder you can go from 300 to even 400mm without noticing much. It's the really short barrels that matter, apparently. I don't know this out of experience but afyer emailing RealSword and ASG asking about it.

 

 

 

Sent from my C6603 using Tapatalk

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I undestand what you mean Lone_Bullet, I have been using full cylinders for barrels ranging from 300mm to 455mm and haven't noticed much of a difference, although I have to admit my eye is quite untrained on these matters, but I think I have been focusing on exotic mods (flat, rhop, different nubs, yaddaydayada) and neglecting basic stuff like air volume, so it's time I start educating myself on it. My play style is quite focused on the gun maximum range range, so every bit of improvement counts for me =)

 

In this particular case, the cylinder is a non ventilated one being used in a 300mm barrel, so since these cylinders are used on 455mm to 509mm barrels, I wondered if this overvolume could be having an impact on the BB since although it flies straight for most of the path, it tends to go off track at the end of it. So I thought I should ask to more capable people =)

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Ye olde volume charts range from 1.5 to 2.5 ratio's.....

 

1:1.5 is too low & explained that short barrels - MP5k's are often seriously overvolumed

but still fire ok for their intended purpose

 

Me personally I feel a ratio of about 1.75 (AEG on 0.20's) to 2.25 (DMR's on 0.40's) is about correct

I have known people with v2.5 gearboxes in SR25's (proper v2.5's not G&G's as they use reg v2')

I've known SR25's to stick with a 510mm barrel even though they could use a 600mm or more barrel

(but chose 510mm as longer barrel length does not guarantee higher accuracy plus higher volume on 510mm for lifting heavier ammo)

 

At about the 363mm & above AEG length some manufacturers start chucking in a full cylinder

(though I feel the poor quality stock seals out of the box compensate for any overvolume on a M4A1 with full cylinder)

 

I previously pointed out the very short barrels but said not to worry about overvoluming on a 300mm:

 

 

So somewhere between 45 to 50 is fine

Don't worry about over volume on medium/longer barrels so much if using .28's

(you have to add on a fair bit to the stroke to push vol ratio over the guidelines)

 

 

The OP asked so I replied and still believe he needs a 45mm port at the bare min

I'd go anywhere in the range of 45 to 50mm so that cylinder can still fill quickly with air through port

Yes o-ring if working correctly should allow cylinder to fill and perhaps different nozzles with prevent " bb suck " on full cylinders

but if you have a port then why not use it to ensure rapid filling of cylinder - well unless you have shortstroked the piston a lot

(but if you have shortstroked the longer barrel gun way too much then you might be undervoluming anyway)

 

All I did was reply with a quick and easy ready reckoner or way of checking cylinder/barrel matching

This does correlate quite well if you wish to do the maths & stuff

It is not set in stone - you can use what you wish as often each build is slightly unique or the results can sometimes vary

 

OP could likely use a 3/4 or 4/5 cylinder but this may vary as explained as these fractions are not quite as accurate as a 5 sec measure of port

OP can use a full cylinder - though I would perhaps stick with a 4/5 one so the cylinder will always fill through ports

 

no worries only trying to help

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