cylinder to barrel volume ratio

[midnight_sailor]

I'm not sure if I actually heard this or not, but does having a cylinder push out a larger volume of air than that of the inner barrel adversely affect accuracy? For example, I have an sr25k with the long sr25 cylinder but only a 400mm 6.03 barrel. What length barrel would I need to get optimum performance in conjunction to that sr25 cylinder?

[Stealthbomber]

Erm, it's easy to explain but harder to provide a concise answer.

 

Firstly, yes. Having a cylinder too big for your barrel probably does have a negative effect on the way your gun shoots.

There are various reasons for this related to air turbulance behind the BB.

Beyond that, people have carried out tests and found that (basically) whacking the biggest cylinder and strongest spring in a gun doesn't always guve you the best power OR accuracy. All a bit subjective but evidence nonetheless.

Lastly, there's empirical evidence. TM make toys. If they thought they could get away with producing a single cylinder and fitting it to all buns they would have. If the felt strongly enough to spend extra money on the tools to cut the slots in a cylinder they must have had a good reason for it.

 

First a simple bit of maths (using guessed numbers cos I don't have a cylinder here to measure right now):-

 

V=pi x R² x L is the formula for solving the volume of a cylinder.

 

V = volume in cubic cm.

pi = 3.1415927

cylinder diameter = 2.2 cm (therefore RADIUS = 0.5 x diameter or 1.1 cm)

Piston stroke = 4.8 cm

 

Note that the piston stroke is the important measurement (rather than the actual cylinder volume) and it should only be calculated for the portion of the stroke in front of any port on the cylinder.

 

Note also that multipying a number by itself is the same as squaring it.

 

So....

 

V = 3.14 x 1.1 x 1.1 x 4.8

 

V=18.24 cubic cm

 

Now we need to do the same for the barrel.

 

V = volume in cubic cm.

pi = 3.1415927

Barrel diameter = 0.604 cm (therefore RADIUS = 0.5 x diameter or 0.302 cm)

Barrel length = 40 cm

 

Note that we MUST use common number types in our calcs, thus a 6.04mm barrel is descrbes as 0.604 cm diameter.

 

So....

 

V = 3.14 x 0.302 x 0.302 x 40

 

V=11.46 cubic cm

 

Remember, you MUST do this calc for yourself, using the ID of your cylinder and barrel and the length of your piston stroke and barrel. The figures above are ONLY AN EXAMPLE.

 

Well, that's the easy part.

 

The hard part is what to do next.

If you look at most guns you'll find that the cylinder volume is significantly more than the barrel volume.

It's common practice for the cylinder volume to be anywhere from 1.25 x to 1.5 x the barrel volume.

This is done to compensate for any loss of air around the BB as it travels down the barrel.

The bigger the BBs you use and the tighter your barrel the closer you can get to a 1:1 ratio between cylinder and barrel although I'd suggest you never go for less than 1.25 : 1.

 

Sooo....

 

Using the above figures (and remembering that they are just an example):-

 

Cylinder Volume = 18.24 cubic cm.

minimum ratio = 1.25 : 1

Therefore

Maximum barrel volume = 18.24 / 1.25

Maximum barrel volume = 14.59 cubic cm

 

Now we need to transpose the equation to figure out what that is as a length.

 

We'll assume we're talking about a 6.04mm (or 0.604cm) diameter barrel.

 

Length = Volume / radius² /pi

Length = 14.59 / 0.302 x 0.302 / 3.1415927

Length = 50.94 cm

 

So, theoretically, if you had a cylinder with a volume of 18.24 cc, the longest barrel you'd want to use would be 50.94 cm

 

 

That's a;ll theoretical.

In practice you might find that a shorter cylinder fitted with a stronger spring offers the same power as a bigger cylinder and a weaker spring.

Figuring out what works best for you might take some experimentation but, hopefully, the above calcs will give you a starting point.

[EmptyChambers]

Short Answer: It doesn't matter if you're stuck wit the elongated cylinder anyways.

 

The difference will be fairly marginal in any case.

[Stealthbomber]

Short Answer: It doesn't matter if you're stuck wit the elongated cylinder anyways.

 

The difference will be fairly marginal in any case.

And yet, as I said, Marui bothered to make different volume cylinders when they could have just used a full cylinder in all their guns?

 

Perhaps they're just gradually getting through a huge job-lot of brass tube that's already got holes in it?

[EmptyChambers]

400mm is not a tremendous difference from 509. As I mentioned, the actual difference in performance will be minimal at best.

 

Its not scientific, but I've run multiple cylinder and barrel setups, largely because I've had primarily P90s and AUGs that facilitated this. I never experienced much of a disparity in the numbers I recorded. +/- 10fps either way, until you got something stupid like a 509mm barrel and a P90 cylinder.

[Stealthbomber]

Its not scientific, but I've run multiple cylinder and barrel setups, largely because I've had primarily P90s and AUGs that facilitated this. I never experienced much of a disparity in the numbers I recorded. +/- 10fps either way, until you got something stupid like a 509mm barrel and a P90 cylinder.

Must admit, I have had similar results.

I tried a bunch of different barrels in my P90 (all using the same cylinder) and noted very little difference in accuracy. I then tried different cylinders and noted a small increase in power but nothing obviously damaging to accuracy.

 

Must say, this is currently one of that last things about airsoft that still gives me the heebie-jeebies.

I mean, as I said, TM are only making "Toy guns". You'd think they would have just stuck a long cylinder in every gun if they thought they could get away with it.

The fact that they went to the effort of machining a bunch of different holes in their cylinders suggest that they had a bloody good reason for doing it and yet I've never really noticed anything startlingly obvious.

[midnight_sailor]

Wow, thanks for all the in depth explanations and user experiences. I guess that saves me some money from having to buy a better inner barrel then, although it is a bit discouraging that I can't make any further improvements to my accuracy.

 

On a side note seeing as this is all for building up my dmr, I'd like to know how many rounds would be needed to break in a guarder 70 degree bucking at 430 fps using .28's.

 

Also for anyone interested in the scs nub, I just got some and I must say that it has improved my shot to shot consistency compared to both big-out and element h-nubs, especially during the end behavior of the bb's flight.

[AnotherJesus]

I don't know about the A&K or G&P or G&G sr25's. In relation to the CA though. The CA has a very long cylinder compared to normal, for no real reason I can see. Not a terribly long barrel either, and even with a massive spring, it leads to poor FPS and loud noises happening. It's prone to poor matching. The easiest way around this, is to use heavier ammo. It's been tested, and heavy ammo has been seen to get much more energy out of the same set up than the light ammo ever did. With an M120 a .2 would get around 340fps. Using a .36 it would get around 300fps. That equates to 1.1J, compared to 1.5J. The noise was dampened, and the flight lengthened. Unfortunately it was ICS .2s, against Digicon .36s. So any accuracy comparison would be meaningless. I think the .36s were worse up till about 30-35m. Can't be sure though, was a long while ago.

 

So, if you have a long cylinder and short barrel, try heavier ammo. See if that helps.