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Cylinder:barrel ratios explained

169K views 216 replies 58 participants last post by  SiliconeSword 
#1 · (Edited)
Hey folks,

Lately it has become clear to me that a lot of people wanting to tech their rifles, mostly new players, don't really understand the whole thing behind matching cylinder:barrel ratios. This is bad, because with a bad cylinder:barrel ratio you have an inefficient setup. This means you need a more powerful spring for the same fps, you're losing shot-to-shot consistency and if your ratio is too high your rifle will also make significantly more noise.

There are some guides out there or posts that roughly explain the idea behind cylinder:barrel ratios, but this guide is intended to fully explain everything you need to make your rifle optimal, along with the theory behind it. I'll use a different colour for theory you don't need so you can skip it if you wish to.

The reason I'm writing this post is to have some sort of compendium containing everything you should know about this subject. That way I can just link people to this thread in the future rather than explaining the same thing over and over again :hehe:

Cylinder:barrel ratio



The cylinder:barrel ratio is a ratio between two volumes, the cylinder volume and the barrel volume. You take all the volume in the cylinder (all the air that's being compressed) and compare it to the volume in the barrel (the area in which it expands behind the bb). If your ratio were 1:1 and your air seal were perfect (no loss of air), then you would end up with zero pressure behind the bb at the moment it leaves the barrel. Intuitively you could say this is what you want, but this is wrong. Your cylinder:barrel ratio should always be higher. Generally something like 2:1 or even higher.

For those of you wondering how to measure the cylinder volume, take the cylinder and measure the diameter (on the inside). Then measure the length (on the inside) from the back side of the cylinder head all the way to the front tip of the piston head with the piston all the way back. If your cylinder has ports (which it shouldn't, we're a sniper forum, everyone here has heavy bb's and ought to be using full cylinders) then measure the length from the cylinder head to the front of the port. The volume is equal to (diameter/2)^2 * length * pi.

The barrel volume is the same idea, but then you take the barrel diameter (6mm) and the barrel length.

So, what's the best cylinder:barrel ratio? That's a question I sometimes see asked, but the answer is that there isn't. The optimal cylinder:barrel ratio depends on a lot of factors, most notably the bb weight. The optimal cylinder:barrel ratio depends (mostly) on your bb's mass.

[There's an optimal barrel length for every configuration (depending on cylinder volume, bore size, bb weight, bb diameter, spring power, air seal, amount of hop applied and probably a couple of other factors). It is really impossible to calculate exactly what barrel length you should use, just like it is impossible to achieve 100% efficiency. There is a point in the bb's path inside the barrel where the force behind the bb (due to the air pressure) is equal to the total drag force the bb is experiencing (at that point). This is the "maximum efficiency point", and it depends on all those factors. Ideally, for maximum efficiency and quietness, your barrel should stop there. This length very much depends on the bb's mass, because a lighter bb will accelerate faster, lose less pressure, and have covered more distance in the meanwhile. By the time a lighter bb reaches the maximum efficiency point it'll be further down the barrel. For a heavier bb the reverse is true, it'll accelerate slower and cover less distance in more time, which means the maximum efficiency point is earlier than the lighter bb. Of course, the heavy bb loses more pressure because it's inside the barrel longer, but it does so over less distance than the lighter bb so the pressurized air is contained in a smaller volume. According to the universal gas law that means the end pressure is higher. With a same barrel length for both bb's, the light bb will generally have left the barrel before the maximum efficiency point. For the heavy bb this is sometimes also the case, but it's closer, so it'll have used up more of the energy that was in the cylinder.]

Heavy bb's benefit from a shorter barrel compared to light bb's with the same cylinder. In other words, heavy bb's benefit from a high cylinder:barrel ratio.

So, what cylinder:barrel ratio should you use for a given bb weight? Again, depends on multiple factors. Lighter bb's generally stick with 2:1 while heavier bb's can go as far as up to 3:1. .66g super-heavy bb's could go even higher but I have no experience or knowledge on that subject. If you're using .66g bb's I reckon you know what you're doing so you won't need this guide.

Bore size influence



Some people like to install a wider bore barrel. The theory behind this is that the bb rides on a cushion of air, which means less bumping into barrel imperfections, a straighter flight path and more shot-to-shot consistency. The tight- vs widebore barrel debate is still going on, and I'm not going into that here, but if you do install a widebore, this has an effect on your cylinder:barrel ratio.

The barrel volume becomes marginally bigger, but that's not the big deal. The big deal is that a lot of air starts escaping. You will need more air or a shorter barrel, because the bore size affects the efficiency of a cylinder:barrel ratio. A widebore will need a higher cylinder:barrel ratio compared to a tightbore.

[Basically, the cylinder:barrel ratio should be a cylinder-volume:barrel-length:bore-size ratio. The barrel volume can chance in two ways, by changing the length or the diameter. But, if the diameter changes, the length of the bb's path inside the barrel doesn't change. What does change, is that much more air is lost because there's a bigger gap between the bb and the barrel. A 6.23mm barrel compared to a 6.03mm barrel can make quite a difference. This is of course not a linear change, but goes exponentially because a bigger bore results in an even bigger loss of air, just because the laws of physics don't want to make things easy. Therefore it is wrong to think in cylinder:barrel volume, you should be comparing the cylinder volume to the barrel length. For most tightbores (6.01-6.05) the results will be approximately the same, but if you start installing a widebore you will see a serious difference in cylinder:barrel ratio efficiency. Lots of air escapes, which means less pressure behind the bb, which changes the maximum efficiency point (as explained earlier) which in turn changes the cylinder:barrel ratio you should be looking for for a given bb mass.]

I don't have any numbers here, but if you do install anything wider than a 6.05, be sure to get a bigger cylinder:barrel ratio. For example .3 bigger.

Cylinder:barrel ratios and joule creep



You've probably heard about the phenomenon known as "joule creep" sometime. Basically it means that you chrono a gun at a certain amount of muzzle energy (say, 1J), then put different mass (usually heavier) bb's in it, and the muzzle energy suddenly rises! You could for example be shooting 1J with .2g bb's and then 1.5J with .4g bb's.

The cylinder:barrel ratio can be used to explain this phenomenon. Like I said earlier, heavy bb's benefit from a high cylinder:barrel ratio, while the reverse is true for light bb's. When you change your bb weight, you change your optimal cylinder:barrel ratio. Your optimal ratio could come closer to your real cylinder:barrel ratio, which means more efficiency. More energy is put into the bb, resulting in an increased muzzle energy.

A common scenario is where a rifle with a high cylinder:barrel ratio is chrono'ed with .2g bb's. The cylinder barrel ratio could be 3:1 or something, which is not very suitable for .2g bb's. They'll get flung out way before they've used up more energy. When you put .4g bb's in, that 3:1 ratio becomes much more favourable, leading to much more muzzle energy.

[Bore size has an influence here as well. Since heavier bb's are inside the barrel for a longer time, more air will escape. In order to prevent this, you can use a tight bore barrel. A high cylinder:barrel ratio with a tight bore will lead to the most joule creep. A low cylinder:barrel ratio with a wide bore will probably lead to the opposite of joule creep, because most air escapes and the bb actually starts decellerating because it has gotten past its maximum efficiency point.]

Adjusting the ratio

There are four main ways to change your ratio: changing the barrel length, the cylinder (effective) length (porting), changing the piston length or using an airbrake. They all improve efficiency because the correct amount of air is supplied to the barrel and all slightly improve accuracy, but the mechanics used and the effects are different:

-Change barrel length (easiest, preferred), better efficiency so lighter bolt pull.
-Port cylinder, better acceleration and compression so lighter bolt pull.
-Install long piston, short strokes the piston movement, better acceleration and shorter bolt pull but not same pull force.
-Use airbrake, less air to the barrel to get it matched, excess air is used to brake piston and decrease noise.

Changing the barrels length is the most common and preferred method as it is the easiest and provides the lightest bolt pull, while all other methods end up putting energy anywhere other than the bb :tup:

What if I get it wrong?

If your cylinder:barrel ratio is not optimal, don't panic, that's no disaster. It just means you're not running at maximum efficiency. You're losing some shot-to-shot consistency but that's marginal. You will probably need a more powerful spring than you should need.

If your ratio is too high, you will have pressure behind the bb when it leaves the barrel. Of course, this means your setup is not very efficient, but this pressure behind the bb as it leaves the barrel also leads to a distinctive "bang" sound, because all that pressure is released. Thus a high cylinder:barrel ratio leads to fps loss and increased muzzle noise.

If your ratio is too low, you won't have enough pressure behind the bb and it will start decellerating inside the barrel before it can leave. This also leads to instability, which means you'll lose shot-to-shot consistency. Your rifle will be more quiet, but you should avoid this. A high cylinder:barrel ratio will just lead to a lot of noise, but a low cylinder:barrel ratio can mess up your accuracy as well. Both lead to reduced fps.

Of course, the closer you get to your ideal ratio, the less significant these things become.

[A bad cylinder:barrel ratio will always lead to reduced accuracy. With a low ratio, your bb will get unstable, lose the "cushion of air" (which is still very much present in tightbore barrels as well!) and it can bounce into the barrel. This leads to reduced consistency and more fliers. A high ratio means there's still pressure behind the bb when it leaves the barrel. As soon as it leaves, the pressure suddenly goes everywhere. Because this is never consistent, you will lose consistency as the bb is pushed around by, what actually is, a small explosion (also the source of the "bang" sound). With an ideal ratio, the air flow around the bb has the same velocity as the bb itself, so this doesn't happen. These things only matter in high-precision builds though, you can still get decent accuracy without matching cylinder:barrel ratios, but as with everything when it comes to airsoft accuracy: consistency = accuracy.]

Other factors

Some other factors also come into play, but they're not as important or they're simply not predictable. Ideally, you should have a cylinder-volume:barrel-length:bore-size:bb-mass:spring-power:airseal-efficiency:bb-diameter ratio, but to keep it simple we're not going to make it that complex :hehe:

One thing that you need to keep in mind is that the air seal is important. If you don't have a 100% (or near 100%) seal then the optimal ratio will change. If you don't have a perfect seal then your ratio will need to be higher because more air will escape. You'll also have less joule creep.

Useful info

I mentioned a "cushion of air" twice. This isn't really the same subject, but I ought to clarify how the bb travels inside the barrel...

The bb is stabilized by a cushion of air, either all around the barrel (the bb is centered in the middle) or below the bb (the bb is pressed to the top of the barrel). Which case is actually happening is a debated subject so I'm not going to go into that here. What is important is that the bb is stabilized by this cushion of air and the absence of this cushion of air is bad for the consistency of your shots. Which is why a barrel shouldn't be too long.

Another important factor is the bb stabilization inside the barrel. It takes some length (say, 100mm, depending on fps) for the bb to stabilize inside the barrel. If the barrel ends before the bb is stabilized it will have bad effects on accuracy. But if your barrel is this short then something weird is going on anyway, so I wouldn't worry about it too much. Just don't go shorter than 200mm, use a little common sense :hehe:

Here you can calculate your cylinder:barrel ratio:
AirsoftTech.dk - Calculator to calculate, Speed, Rate of Fire, Gearsets, etc.

1tonne has written a couple of great guides, and there's a section with ratios (& barrel lengths) for the VSR platform. This contains all the ratios as tested by 1tonne and should be correct ratios (for given bb masses), but of course, with the amount of factors that come into play your optimal ratio could be different.
http://www.airsoftsniperforum.com/41-vsr-rifles/6075-advanced-vsr-sniper-building-guide.html

And here's a rough indication of what (AEG) cylinders you should use with which barrel lengths. Keep in mind that this assumes "normal" barrels, so no tightbores, and standard bb's. If you put heavier bb's in it you will want to change your ratio. These are for approximately 2:1.
FA: Florida Airsoft - Volumetric Ratio Chart

There is a calculator made by HS5 (edited by Boesboes) that can also calculate your cylinder:barrel ratios along with some other things (bore up compensation), although I'm not aware of which calculations it uses so I can't guarantee its accuracy :hehe:
If you want access to this calculator you should send BoesBoes a PM, I deleted the link on his request.

Disclaimer
Because everyone and a couple of other people seems to claim this whole post is plagiarism and I've been catching a fair amount of hate for it, I ought to add this. Most of the things I described here were not invented by me, I learned them from other people on this and other forums. This is merely a guide explaining how it works and is not a scientific paper published by me. If anything, I'm only compiling a bunch of posts to make it easier to read and understand for new people. Someone suggested I look up "plagiarism" in the dictionary, and I did. Plagiarism is using someone else's work and pretending it is your own. I am using someone else's work in the broad sense, but then again... isn't everyone who's writing forum posts? I am not, however, pretending it is my own. If anyone disagrees with me, man up and tell me what's wrong so we can work it out.

Cheers, :)
Reliku
 
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#33 ·
From what I understand, it would be very difficult to make this rifle into a JC rifle. In fact, it will almost always either lose joules or only gain a very, very little energy by using a heavy weight bb. It may gain a little if you use light weight bb's such as 0.25gm or 0.28gm (possibly gain).
 
#34 ·
I don't know if it has been asked before, so I apologise beforehand.
Maybe I am hard headed, but I can't seem to understand what bb weight to use for a specific ratio. Here are the details: AUG A1 made for DMR, 455 mm inner barrel, full cylinder, bore-up kit, 128 m/s (~420 fps). Only Saturday I will be able to measure the diameters, but I want to understand what bb weight to use.
So, from 1tonne's guide and Florida Airsoft chart I should use 0.3 g bbs for 420 fps with 554 mm inner barrel or my current barrel length.
Or, if I'm not yet clear, for a specific ratio (let's say 2.96:1), where does the bb weight enter in Reliku's ecuation? What weight to use for this ratio?
Thanks.
 
#35 ·
1tonne's guide is for VSR cylinders, keep that difference in mind :tup:

Weight for different ratios remains some guess-work. 2.96:1 is pretty high, I'd say the optimal mass for that ratio is somewhere between .4-.45, maybe even higher.

It depends on many factors, so it is impossible to get perfectly right just by doing calculations, but you can make a very educated guess.

What's your cylinder volume with the bore-up kit? Divide that by your barrel volume and you get a certain ratio (let's say 2.5:1), which would be a nice ratio for something like .36g.

But considering the many factors to come into play, your gun may be shooting most efficiently with .4g or .3g instead. But .36g would be a good place to start. Assuming the ratio is 2.5:1, of course :tup:
 
#36 ·
Thanks a lot, Reliku. That's exactly what I wanted to know, where to start and aproximately an interval to work with. Currently I use 0.28g, but only in the weekend I will know precisely what're the inner diameters. So, if I will have something like 2:1 - 2.2:1, I should stick to my 0.28s or maybe 0.3. It'll be a lot of trial and error, I suppose.
 
#37 ·
.28 for 2.2:1 sounds OK, but I'd go with a slightly higher ratio and heavier bb's for 420 fps :tup:

Rule of thumb: 2:1 for .2g, 3:1 for .45 or so, 4:1 for .66g and heavier. Finding the ratio to start off with is a bit guesswork, if you want to be absolutely sure you'll just have to test your gun's muzzle energy with a bunch of different bb's :hehe:
 
#38 · (Edited)
I've been doing some calculations about the pressures and energies involved in spring powered guns, and some interesting things are beginning to appear:
- The space inside the cylinder head is important.
- The best ratios may somehow be related to the constant e.

I don't really know enough physics to conclude anything, but it has been sort of interesting.

Someone said something about HS5's formula for the ratios. It's this:

ratio = 0.29E + 1.37 where E is energy in joules

The reason why the Excel formula looks complicated is because it converts fps to joules, which requires converting fps to meters per second and putting that into the equation E = ½ mv².
 
#39 ·
I'm certain the muzzle energy has something to do with it, but it is nowhere near the only factor. The problem is that so many things have an influence, it's hard to come up with an exact formula :shrug:

I don't think HS5 was correct on this, because that would mean there is no difference in efficiency if you use different weight bb's (to state the most straightforward example), which is obviously not true :yup:
 
#40 ·
When i had adquire the VFC M40A3 i made the JC, JC is related to ratios volume.

With stock gun 427 fps (0.2g), the best ammo was .30g with 2 fps gain (351 fps).

When i opened the gun , i´ve made the piston to act like a piston ring on the combustion engine, to create maximum pressure/ compression (2.5 bar) without compromising the piston movement. The ratio is the same, only switch some internals with the exception of the spring its the same.

Now, the gun have 350 fps (0.2g), it still is the .30g but with gain of 28 fps (314 fps).

VFC M40A3 - 2.79:1
 
#41 ·


Always wondered about this, is the spring guide stopper section in the middle of the picture be considered a port? I get the idea that if it there was less of that cut out it would have more compressed air or something like that.. :shrug:

Razinger | The one, The only..
 
#42 ·
Yes it is a port. It can only be shortened by about 1mm. If you were to shorten it further, the trigger sear cannot catch onto the piston.
 
#44 · (Edited)
This post is very technical but I think it explains the relationship between energy, bb mass and the correct cylinder:barrel ratio. It shows how the energy per unit mass (joules of the gun divided by weight of the bb) corresponds to the correct ratio by a factor of 0.40.

It seemed odd to me that the correct barrel:cylinder ratio would get larger for higher masses of bb. That is, why would a heavier bb require a shorter barrel? The ratio must depend on some quantity that gets smaller, not larger, with heavier bbs.

The energy per unit mass is a quantity that does get smaller with heavier bbs. So:

fps@0.20 g energy (J) correct mass energy/unit mass (J/g) correct ratio (cylinder:barrel, C:B)

fps E(J) m(g) J/g C:B
420 1.65 0.30 5.5 2.26 0.41
450 1.88 0.32 5.9 2.36 0.40
500 2.32 0.36 6.4 2.51 0.39
540 2.71 0.40 6.8 2.67 0.39
580 3.13 0.43 7.3 2.92 0.43

fps@0.20, correct mass and correct ratio are taken from 1tonne's Advanced VSR Building Guide. The guide instructs you to use your fps at 0.20 g to determine what bb mass to use then provides the correct ratio for each mass. I converted fps@0.20 to joules. Then I divided the joules by the 'correct mass' to give energy per unit mass. Dividing the energy per unit mass by 1tonne's correct ratio gives 0.40.

This is not really useful because all the common masses already have a ratio and the 'correct' barrel is too long to be useful for 0.28 and below. But it does show that there's some relationship that explains why heavier bbs benefit from shorter barrels. 0.40 is close to 1/e (0.37). That might be coincidental but I think it's not. See Harmonic for why... the ideal barrel length would be 1+1/2x+1/3x+1/4x...1/infinity x (the definition of e).

The factor is close to 1/e and not e because we are using cylinder-to-barrel ratios, numbers greater than 1. If you used barrel-to-cylinder ratios, you would get numbers smaller than 1, and the correct ratio would approach e, 2.718.

Example: For 620 fps with 0.20 g and a mass of 0.45 g, the energy is about 3.45 J and the energy per unit mass is 7.7 J/g. Multiplying this by 0.4 gives 3.1. The guide suggests a ratio of 3.14.
 
#45 ·
Interesting idea. I'll play around with it for a bit, I'll do some calculations to come up with a table (and possibly some testing). If this theory is correct then it would provide a way of calculating the ratio for other masses as well (solely as a function of bb mass).

That being said... The ratio does not solely depend on the bb's mass, so 1/e will definitely not be the only factor (if it is at all true).

I believe HS5 came up with some formula for ratios as well, which basically claimed that the ratio depends on muzzle velocity. We know nowadays that that is not true, because there are MANY more factors involved :hehe:

Still, interesting idea, thanks for posting it :cheers:
 
#46 ·
There is already a thread where someone had a chart on here. Not sure what thread it is though. It was a couple of years ago.
I will have a look when I get some time tomorrow.
 
#47 · (Edited)
Well, the formula produces ridiculously long barrels for weights below 0.3 g, if the cylinder remains the same at about 35 cm^3.

The idea is that the cylinder:barrel ratio is related to the energy:mass ratio. Here's a graph:



Energy to mass ratio, that is the muzzle energy divided by the mass of the bb, is probably a better way of saying it than 'energy per unit mass'.
 
#48 ·
That theory is flawed then, because the strength of the spring does not determine the energy. It has influence (pretty obvious) but many more factors apply. Which is exactly why people keep playing around with different ratios and different setups. Joule creep is a prime example of why sping power =/= muzzle energy.

Therefore I doubt the validity of the whole theory, too many factors apply to say whether this is a consistent relation between the two or just coincidence.

However, the graph you posted is an interesting observation and it does hint at a relation between the c/b ratio and the energy/mass ratio, so it's definitely interesting to investigate. But I don't think it'll provide an accurate formula. At best you can make an educated guess about the best ratio, which is something we already have: 1tonne's chart on ratios vs bb weight, which is in pretty much all cases plenty accurate :tup:

1tonne, I meant something like the graph RKS posted, I haven't seen such a theory before, but I may have missed it :shrug:
 
#49 ·
I have tried searching but could not find the graph I talked about earlier. The thread would be in 2013. Maybe early 2014. It did have some faults but seemed reasonably OK.
 
#51 ·
I changed 'strength of the spring' to 'muzzle energy' because, as you said, there's more to muzzle energy than the strength of the spring.

As mentioned there are already some ideal ratios -- actually, the 0.40 formula just predicts those ratios. I think there might be other numbers besides 0.40 that work, simply because 0.40 requires absurdly long barrels for anything lighter than 0.3 g.
 
#53 ·
Dude, That's one hell of an explanation. And it's not plagiarism, either--if that were the case, nothing we write would be original and we'd all be in trouble.

I'm a math rock so I can't begin to calculate or apply the physics behind your equation. However, I wanted to say that I appreciate that guys like you exist who can help noobs like me with basic and advanced questions.
 
#54 ·
I've got a few questions since I'm new to the game and don't understand the exact science behind joules, energy and BB flight. Please bear with me as I explain my concept of energy buildup and distribution

I bought the UTG MK96 because I heard it was great stock out of the box. No complaints there. I can hit a can of Monster from 120' away about 30% of the time. When I do miss, it's usually to the left or right by a few inches. I bought the PDI hop (over AA) specifically for the dual arms to eliminate the stray left and right shots.

With this in mind, I bought a PDI barrel for brand compatibility. I chose the 495mm bull style barrel for increased consistency (less vibration) and opted for the 6.01 mm so the BB shot straighter when exiting the barrel. My logic is that by going with 6.01, less air escapes around the BB through the longish barrel to compensate for any friction that might build up. Also, I wanted to reduce the risk that more air would unevenly distribute (more air on the top, bottom, right or left) as the BB travelled down the barrel so I stayed away from 6.03-6.05.


My next upgrades will be my cylinder and piston/spring/guide. In order to be effective, I know I need a decent cylinder with a snug fitting piston and a tight fitting cylinder head.

1. Am I correct that to maximize BB output, I need to have a strong enough spring to accelerate the piston (but not destroy the trigger box) with little to no air pressure escaping through the piston, cylinder head, hopup and barrel? This is a bare-bones explanation of what makes sense to me without ratios, joules and other formulae.

2. Am I also correct that the more energy (air pressure) that builds up, the heavier the BB I need to stabilize my long distance shot? I think I read that somewhere in this thread. I use .32 in my UTG now and notice a considerable difference from .25.

3. Also, how do I maximize my accuracy at distance without exceeding @550 fps? It would be counterproductive having an awesome rifle that is banned from fields. I need the layman's guide to air pressure efficiency and consistency.

4. Do you guys have any suggestions for a cylinder, cylinder head (or cylinder with a head), piston, spring and guide so that I can maximize my distance and accuracy. Kenteroo's setup looks to be efficient since he has the same brand but I'd like others to weigh in, too.

Thanks for bearing with my simplified logic and request for advice.
 
#55 ·
Basically, you are correct. The power of the spring you need depends on the desired fps and the airseal in the whole system, but it also depends on the ratio (sorry, for true 100% efficiency you have to take the ratio in account :hehe: ). Your barrel is a tad long for a type 96 cylinder but it should work with .3-.36.

As for bb weight, yes, higher energy means you'll benefit from a heavier bb. It doesn't make sense to use .25 g bb's in a 500+ fps gun, but people keep doing it for some weird reason...

The key to long range accuracy is using heavy bb's of high quality. You also need high quality parts (barrel group mainly), but that's kind of a no brainer. Fps isn't that important (500 is plenty enough) :tup:

As for cylinder/piston etc, it doesn't matter too much, it just needs to have 100% seal. If you have the budget available, PDI makes great parts :tup;
 
#56 · (Edited)
Hey I was hoping for a little clarity. I have a aeg SVD with a v2.5 cylinder that's combined with a 650 mm 6.03 mad bull barrel. I'm going to r hop a new barrel that isn't Teflon coated and I know I need a shorter barrel. Right now my gun is 420-425 fps with .2s. I normally float between .28 and .30 buy I'd be willing to use heavier bbs. Can anybody help advise me on a barrel to get in order to help fulfill the dmr role. I was thinking 6.04 590mm maybe?

Edit: that's with a psg1 200% spring. I'd like to get it up to 450 with .2s,which might involve getting a 300% spring and cutting a few coils. But that's second priority
 
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