Cylinder /barrel ratio. Test and results.

[Hangtight]

I've just spent an interesting couple of hours cutting a barrel down 30mm at a time to try and get a proper answer to the correct cylinder /barrel ratio for a few different BB weights, and I thought I'd share them here.

The test was done with a single gun, using an M100 spring and a cylinder with a 12mm port for a compression distance of 48mm in a 23.8mm bore cylinder. I cut and roughly crowned the barrel by 30mm each time. It was not disassembled from the hop, and neither was the hop setting changed at any point. I fired a string of twelve shots for each BB weight at each barrel length to give an average and a deviation.

460mm barrel (ratio 1.60:1)

0.2g    356avg, +/- 5

0.25g  324.9avg, +/- 7

0.28g  305.1avg, +/- 3

430mm barrel (ratio 1.73:1)

0.2g   357.1avg,  +/- 5

0.25g  326.6avg,  +/- 4

0.28g  305.8avg,  +/- 2

400mm barrel (ratio 1.85:1)

0.2g  358.7avg,  +/- 3

0.25g  329.0avg,  +/- 3

0.28g, 304.5avg, +/- 1.5

370mm barrel (ratio 2:1)

0.2g  353.6avg, +/- 2

0.25g  322.4avg,  +/-1.5

0.28g 301.9avg,  +/- 1.0

340mm barrel (ratio 2.2:1)

0.2g  348.9avg,  +/- 1.5

0.25g 321.0avg,  +/-1.5

0.28g  301.0avg,  +/-1.0

It would appear that the ratio that gives the best efficiency is not necessarily the same as the one that gives the best consistency. More volume would appear to be better than not enough.

So the numbers I've been quoting for cylinder ratio have been out, but the barrel lengths  I've suggested for say, a full cylinder and 0.28g BBs have been about right. In general, it would seem that a full AOE corrected cylinder is well matched with a 363mm barrel and 0.28g BBs.

I might carry on cutting down the barrel and get some more figures, but it's a nice quality TM one, and now it's about the right length for one of my project guns.

 

[Hangtight]

Couple of extra numbers... Piston weight is 26.1g and there is a bearing on both the guide and the piston. There's probably another 10fps in there as this is a well used gun that could really do with better lubrication. It was put together with white lithium grease before I knew any better.

 

[Davegolf]

Interesting, is that a type B cylinder, as in the ports break out the rear of the cylinder?

Still relative results even if the gearbox internal spec could be costing you a few FPS

 

[Beowulf]

Wish you could have started with a longer barrel for snipers

 

[Hangtight]

If anything, sniper rifles actually need shorter barrels. The heavier BBs need a higher cylinder/barrel ratio. For 0.43g, a ratio of 2.9:1 is suggested. Just like AEGs they only have so much cylinder volume, so for the ratio to increase, the barrel length has to be reduced. Some of the most accurate spring sniper rifles have barrels around 300mm.

 

[Davegolf]

Hangtight would have had to start with a sniper rifle though not an AEG??

A VSR type rifle should have a much longer barrel to suit the larger cylinder available to that platform

 

[Sitting Duck]

Soz to crash but have I read the cylinder window (12mm)

is 48mm from front ???

subtract 15 from 48 = 33 & multipily by 10 = 330

330 is a rough guide to matching up barrel lengths to cylinder

(giving you aprox 2.0:1 ratio dooberry give or take a smidge)

NOTE NOT 12MM BUT TAKE OFF 15MM FROM 48MM = 33 OR 330 BARREL AFTER X10

see here but the post I created a while back at the very end I revised the 12mm figure to 15mm

(basically to allow a bit more I found for piston releasing & pressure to start building)

WARNING - NECRO POST ALERT !!!!







Those volume ratio calculators & even the pic of cylinder ports to M4 carbine are a bit confusing

plus the 2/3 or 3/4 or 4/5 bollox is open to interpretation I thought at times - sod measure, minus 15 & x 10

Same as gear ratio malarky counting/pulling teeth - just multply the bevel turns by 3 than f*ck about loads

Not quite 101% of exact volume ratio this  (X -15) x10 method gives out

At time of writing I was guessing @1.6 maybe 1.7 then this forced me to revise to 15 not 12

Still a bit sad looking at things in my OCD way but not quite as bad as I used to

(the medication is at last starting work)

Anyway good guide/test results sir especially the figures and levels of fps variation

nice one 

 

[Esoterick]

If anything, sniper rifles actually need shorter barrels. The heavier BBs need a higher cylinder/barrel ratio. For 0.43g, a ratio of 2.9:1 is suggested. Just like AEGs they only have so much cylinder volume, so for the ratio to increase, the barrel length has to be reduced. Some of the most accurate spring sniper rifles have barrels around 300mm.

This seems contrary to what is commonly recommended on both the VSR owners facebook group and Airsoft sniper forums, which is to use a 430mm barrel even in a G Spec when using weights around 0.40g

 

[Davegolf]

430 is pretty golden on performance for a 500fps BASR.

@Sittingduck that's a cool way of thinking to quickly work out barrel length, I think that 12 was better than 15, actually 11 because 11 gets you in the 2.1/2.2 ratio zone which is best for 330-360fps, gives max joule creep with least spring effort or over volume.

 

[Sitting Duck]

A G&G SR25 has a std cylinder not a proper longer 2.5 cylinder like SVD & L85 19 tooth thingy's

In THEORY they should only be using a 455mm barrel in that sense

But they can use a 510mm if setup REALLY WELL but it is pushing the volume imho

Ideally for say 550 barrels you will need a longer cylinder like the v2.5 SVD L85

But even then I would not go beyond 610 barrel

The volume thingy is still open to debate but the general guide is to stick with an aprox vol ratio

(depends on bb weight but again it will vary a little plus the odd one that works but perhaps pushes limit)

TBH I stopped studying this stuff way too much, it is open to everybody's take and instances that break mould

I got a rough - and I say rough idea or method to avoid doing PhD maths n crap on gears & cylinder/barrels

It seems to kinda work (so far) so I'll still continue to use an easy rough n ready reckoner

on very short (mp5k) or very long (AEG SVD/L85) the correlation may not be 101% perfect but in general it works

(well I think so - so far)

 

[Sitting Duck]

430 is pretty golden on performance for a 500fps BASR.

@Sittingduck that's a cool way of thinking to quickly work out barrel length, I think that 12 was better than 15, actually 11 because 11 gets you in the 2.1/2.2 ratio zone which is best for 330-360fps, gives max joule creep with least spring effort or over volume.

It depends on a few factors - you can correct the AoE perfectly or slightly over-correct or under-correct

The Correcting of AoE bang on or smidge over/under "can" effect the stroke/volume more than any bore-up crap imho

On the piston's release it is completely static,will need to accelerate to final speed

the o-ring hasn't even begun to seal let alone compression starting to build properly

So there must be a good 5mm of travel before any power is actually generated

Hence I gave it a few more mm's to allow for the power to build from nothing

As I say it open to everybody's take on stuff and stuff will vary

but the maths of cylinder barrel ratios even on a easy to use online airsoft calculator

or a tiny little app is open to variations or interpretation or perhaps I think hmmm nahhh

I just noticed there was some correlation to gear ratio checking

and also the dimension of the port to barrel it is supposed to be paired with

tweak it all how you see fit but just gave a rough basic guide to consider as a foundation perhaps

nothing I ever say should be taken as 101% Gospel as I often talk out of my ar$e

Plus we are ALL still learning with every build - so nothing I say is set in stone

 

[Davegolf]

Totally, for the most part any additional AoE will decrease your swept volume.

Youve probably noticed that unless you run a long ass barrel a full cylinder normally reduces FPS because the the piston cannot get a 'run up' to gain some speed before meeting cylinder pressure resistance.

Ive found where full volume is needed a type B usually out performs a type A, whether that's down to faster piston acceleration or better piston sealing early in the strike due to the more aggressive cylinder pressure who knows.

like you I can't be arsed with the math but trial and error proves what works

 

[Hangtight]

Total piston stroke after AOE adjusted is 60mm. The port is 12mm, so the piston only compresses air for a stroke of 48mm.

Working out the ratio for each BB weight at the point where the fps variation gets really nice and small, I get 0.2g 2:1, 0.25g 2.2:1, 0.3g 2.4:1 (extrapolating a bit). This pretty much matches as a linear increase with the suggested ratio of 2.8:1 for 0.4g in spring sniper rifles. Which is about 450mm in a VSR and just over 400mm in a L96 (been a long time, had to go and check. )

 

[Davegolf]

Yeh those ratios per weight are the perfect starting point and get you so close to where you need to be, then if you want to / can be arsed to chrono at your desired BB weight vs 0.2 reference to get as much joule creep as possible, that way you know you are transferring the maximum amount of energy into your little ball of shootyness

Currently got my NGRS Scar at 2.17 with 0.28s with no loss of energy from 0.2s

Not only does a gun shoot nice when all is matched it sounds so stress free as there's not tons of extra air blasting out the end of the barrel or the piston hammering the cylinder head because of lack of volume. Satisfying

 

[Hangtight]

That was one of the things that was most noticeable as I shortened the barrel... Just how much the shot cycle smoothed out as you got closer to the 'right' ratio.

I must be doing something right, cos I hit what I aim at, and from far enough away to be accused of cheating!

 

[Sitting Duck]

Long story short....

One day if/when you get a chance, pop the top receiver say

Then take a pic maybe of port with the dimension of front of cylinder to port

Basically the front of cylinder to red line or front end of port please if you can

We can call this distance X before we get into geeky world

Just trying to see if the amount to subtract from X is 10, 12, 15 or so before multiplying by 10

More trying to see if my theory works out and if we can use a rough ready reckoner

Than a calculator or app

A good one is here:

https://www.cynosurex.com/Software/AEG Compression Volume Calculator/

because you can mess about with figures and ratio updates as you adjust crap

The stroke of 60mm might be an absolute max on AoE it could be 58 at a smidge over 12 o'clock

(normally it is about 12:30 position but everybody has their acceptance of total perfection or acception)

A non AoE piston you got say 4mm or so extra stroke making 63 or 64mm stroke

The stroke even just a couple of mm will effect volume possibly more than any bore up kit

(mess about in calculator and see it do its maths magic on exact same gun)

hence to DMR mofo's sinking money into bore up kits, probably best just partially correcting AoE

Anyway - soz for waffle, great test - if you get a chance post a pic with dimensions of port to cylinder head

then I/we might have idea what to take off in a rough ready reckoner (please - with sugar & cherry on top)

 

[Davegolf]

Think it's worth making it clear AoE has nothing to do with volume adjustment primarily, but the more AoE you run wether it's behind the piston (old stylee) or sorbo on the cylinder head (current style) will reduce the available swept volume as a by product of its actual function.

All standard cylinders are 72mm total in length.

Pretty much all cylinder heads take up 6mm of that when fitted.

Now we have 66mm available.

Next a standard piston takes up another 6mm when fully cocked / rearward.

Now we have 60mm available, the maximum swept volume possible with a type 'A' eg 'unported' cylinder.

Any amount of AoE wether it's added to the cylinder head or the piston head will reduce this swept volume.

 

[Davegolf]

Here's my NGRS Scar.

Red line is 53.5mm

(type 'C' ported cylinder)

Blue line is 10mm

(6mm cylinder head with 4mm sorbo protrusion)

So dimension 'X' is 43.5

X minus 11 times 10 is 325

Which is very close IMO to the right barrel length for 2.1 - 2.2 volume ratio @ 330-360fps

But as you say is speculative.

I mean you could strip it right back to a simple rule, barrel length is equal to FPS once you're running at or above 1j / 330fps.

Then use your X tool to determine correct cylinder volume.

You could work out a multiplier for different fps levels and respectively BB weight.

View attachment 30291

 

[Davegolf]

PS I'm proud of the straight finger painted lines

In fact that's a worthy achievement right there I could happily go to bed now knowing the day wasn't wasted 

 

[Sitting Duck]

I would have said 53.5 - 15 = 38.5 x 10 = 385

cylinder port would ok to use for up to 385mm barrel

That was what I did the rough ready for

What I'm trying to do is say we increase the amount to subtract

(originally about 10mm is where the piston head rests on cylinder head)

SAY we did 53.5 - 20 now = 33.5 then x 10 = 335

then this would be the newer setting to use up to a barrel length of 335 with a higher barrel/vol

THIS is why I'm asking Hangtight to display his port position

OR if you can list the length of your barrel in the SCAR perhaps

This way it can refine or improve a rough ready guide for say a decent-ish 2.0:1 guide

without going done the calculation route or trying a shorter barrel to improve the vol

I'm just trying to see if the rough guide subtraction amount or Y or whatever

can helped to be refined that was all

To sum up you took off say 10 where head+sorbo was

then took off 11 then x 10 etc....

What I started with was just 12 only then went to 15 to subtract

there was only really one figure to subtract then x 10 etc.....

(just to keep it as simple as possible I guess)

I then revised this to 15mm (still lower than your 21 (10 then 11 subtraction)

as 12mm was producing say 1.65 ratio so 15mm produces a more 1.8 ratio

Which according to some guides is acceptable but I have reservations....

according to this even 1.42:1 is acceptable but I think that is bollox on PDI AK47

hence me trying to push for 1.75-ish at least or 1.8:1 with revised 15mm number to subtract

now if that needs to increase to (X - 20/25) x 10 = ideal barrel for a 2.0:1 then fair do's

Now with both your gun & Hangtight's testing you are happy with a a ratio of 2.1 - 2.25 ish

Soooooooooooo

if you inform us of your barrel in SCAR

& Hangtight posts the pic/dimensions of his cylinder porting

THEN it might be a straightforward of say 53.5 - 22.5 or something = 310 x 10 = 310mm barrel

eg: sunbtract

15-ish for 1.8:1 ratio

20-ish for 2.0:1 ratio

25-ish for 2.2:1 ratio

see what yields best results in shooting or proof of pudding

then it might provide a fairly accurate rough/ready guide in seconds

than try to enter & calculate all the variables

cylinders don't vary that much and tbb Vs reg decent barrel of say 6.05

or bore up kits do not effect the final vol results THAT much

excluding very wide bores or AEP cylinders most stuff is within a regular margin

but the stroke just by a couple of mm can start to impact ratio a fair bit

so trying to put or refine an easy checky thingy majig based upon port position etc...