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AustinWolv
01-20-2005, 05:31 AM
Some long reading captured from another section of this board, pertaining to how AEG cylinders, barrel length, and BB weights can affect the muzzle energy of a weapon:

Cylinder volume - if the cylinder does not have the internal volume to hold enough air to propel a BB down a certain barrel length, the BB will slow down in the barrel due to not longer being accelerated by a force behind it (air pressure). Conversely, if it has more internal volume than the inner barrel, the BB will not reach full velocity before exiting the barrel because the initial rush of air is what propels the BB, not the full volume being expelled from the cylinder by the piston racing forward. An example would be Obsidian’s G3SAS: It has a very short inner barrel with a type0 cylinder if I recall correctly, so the BB is not reaching the full acceleration before exiting the barrel, so thus he installed a stronger spring to compensate. In other words, he is using a spring that is capable of more energy with parts that stunt that capability, thus allowing that particular gun to be legal. A different approach would be to use the appropriate cylinder for that short of a inner barrel length, which would allow that gun to reach about the same muzzle velocity with a weaker spring, thus saving some wear and tear on other mechbox internals. In those two scenarios, the gun has the same muzzle energy, but went two different routes to get there, with one being optimized for that barrel length and the other using brute force.

Thus, this is why various cylinder types (positioning of holes in the cylinder) are matched with certain barrel lengths, because you want as much air moving as fast as you can get it to be what is expelled through the nozzle.

When an AEG fires, it is expelling a set amount of air. Since the air from the cylinder is at the same pressure as the atmosphere, the BB is only accelerated by the volume of air behind it. This differs from a gas gun, in which a certain amount of gas is expelled or valve simply opened, which allows the propellant gas to expand rapidly and try to equalize with the atmosphere. In that case, the gas will actually cause the BB to gain velocity in the barrel as the gas increases speed in order to escape the barrel to equalize with the atmosphere.

Now in both cases, BB weight can cause various muzzle velocities without changing the spring. This is because a higher-powered gun is forcing air out that much faster. Thus, a light BB might be expelled very quickly by that initial burst and not reach the energy that the gun is actually capable of......in other words, if a heavier BB is used in that same gun, you can actually see a muzzle energy increase because the BB is heavy enough to not be shot forward by that initial burst. This means the full volume of air is behind it moving quickly, and it reaches full acceleration as it exits the barrel. A BB that reaches full acceleration earlier in the barrel will actually slow down as there is no force behind it anymore, so it is traveling on inertia alone and frictional forces, due to air and also the walls of the barrel, act to slow it down. This is relevant because with high-powered sniper rifles, testing is done with a .2g BB. This is faulty because the gun is tuned to shoot with a heavier BB, like .34g or greater, let's say. Thus, that lighter BB may exit the barrel before the full energy potential is reached. For instance, there have been cases where somebody will chrono at 490fps with a .2g BB which equates to 2.2J of energy. Then, they chrono with a .36g BB at 420fps which equates to 2.9J. Same gun, very different muzzle energy because of what I outlined above. That is where muzzle energy comes into play and is a better indicator of a gun's energy capability than pure velocity. You can see how that could come into play at the chrono pit, and a player innocently and unknowingly is wielding a gun that is shooting at higher muzzle energy than expected.

This is usually only seen in higher-powered guns, but can be seen in our regular AEGs as well to a smaller degree. For example, let's say you have a gun chrono at 400fps with .2g BB, which equates to 1.48J. I have seen multiple common instances where that same gun will then chrono at 360-370fps with .25g BB, which equates to 1.49-1.58J. See how the energy rise was slight, although the gun legally shot 400fps with .2g BB?

Thus, the relationship is not linear, but is dependant on parts in your gun, including the nozzle, tightbore barrel, piston head, cylinder, cylinder head and inner barrel length. Furthermore, if those parts aren’t sealing well, that throws a bit more variance into the mix. However, due to experiences with various guns, you can get a feel for where a gun should lie in the muzzle velocity range based on what BB is being used. So your statement about expecting a gun to shoot anywhere from 25-50fps higher with a .2g BB versus a .25g BB, as a loose general number, is accurate, although in my general experience I would typically say 30-40fps is a more likely number. I threw out 25-50fps earlier to incorporate more variables.

The same discussion mentioned above for sniper rifles and regular AEGs also applies to support weapons. For example, one of my RPKs chrono'd at 442-447fps consistently at the last Roanoke op with .2g BB, and prior to going to that op, I had checked it at home with .25g BB to be anywhere from 401-411fps. The .2g BB energy works out to 1.79-1.87J, whereas the .25g BB works out to 1.85-1.94J, so you see how the same gun can produce slight muzzle energy differences just because of the BB.

This ALSO is further complicated by the brand of BBs you use. Some brands are nominally smaller than other brands. For example, I have some KSC .25g BBs that a handful measured in the 5.92-5.96mm diameter range. Compare that to some AE .25g BBs that I measured that fell in at 5.94-5.97mm range. The AE had a smaller tolerance range and a slightly larger diameter, making them more consistent and accurate. This is more apparent in gas guns, in which the hopup systems differ than that of the AEGs, and I am still learning about that having picked up types of those guns in the past couple months.

Ok, I think the only thing I haven't covered is bore-up cylinders.......they have a slightly larger internal diameter for the purpose of being used with longer inner barrels with higher-powered springs. You would not use a bore-up in a short inner barrel gun for the reasons outlined earlier…..the BB would exit the barrel before full acceleration is achieved. I personally am not a big fan of them since they require a matching piston head, and those are not readily available to my knowledge, unless you buy the entire kit again. Therefore, I have less experience with these, but the same basics apply. The thought is that if you have a larger air volume in the cylinder as the source, along with a long inner barrel, that you can achieve greater muzzle velocity since you are allowing the BB to accelerate longer with a larger pressure behind it.

Basically overall, you would pick what inner barrel length you are seeking first, and then pick a cylinder type to match it.


Obsidian weighs in on the same topic, adding his experiences with BU kits:

Long barrels need Boreup Kits:
Since wolv didnt run over bore up cylinders.... bore up cylinders have a thinner cylinder wall as well as a larger bore on the cylinder head nozzle. Due to this larger bore the nozzle must be a larger bore. This basically gets you a minimal increase in volume throughout the whole system. It has the downside of each part being unique and non replaceable for the most part. Going to wolv's explanation I have seen several boreup kits that resulted in a drop of BB velocity. Not what someone wants after spending 50 to 100 bucks on an upgrade. This is due to their being a larger volume of air that the same sized spring is trying to push. So if your gun was shooting 400 with and you put in the boreup kit because you bought into the marketing hype you would probably end up shooting around 385 after. In my opinion they are a waste of money. To prove this I built a g36 up with a RAS and a 650 mm PSG barrel....longest barrel available on the market. I have an M120 and type 0 cylinder installed and the gun shoots consistantly 400 to 405.

You need a long barrel for range/fps/whatever. Barrels give you accuracy and can contribute to range due to the volume that they possess. I constantly hear about everyone wanting to make their barrel longer to get better range or more FPS. As you have seen it is a science. You need to have a barrel that matches the volume of air that your cylinder is pushing out. Your velocity is coming from the compression of the air from your spring. As wolv said if your barrel contains less volume than your cylinder you are wasting some of that compression your spring generates. My G3SAS has one of the shortest barrels out there. It actually has the stock cylinder in it which has a hole in it. With an M120 the gun will only shoot around 350fps. This is because the gun has half the barrel/cylinder volume of an SR16 for example. To compensate for this I put an M130 into the gun. This has the effect of compressing the air faster to generate more pressure. This gets the bb up to speed before it leaves the barrel. This results in it shooting 400 fps. Now lets look at the flip side....Longer barrels. In my opinion the G3 has the perfect length barrel. I have g3 barrels in my P90, SR47, the other G36 and a couple other guns. When you pull the trigger and the piston slams forward compressing that air you get a very violent turbulent reaction. All of this compressed air blasts out and hits the bb pushing it past the hopup which starts it spinning. Some distance down the barrel after that the bb is going to be moving at its terminal velocity. The advantage of a longer barrel or a tightbore barrel is that it holds the BB in that trajectory for a longer time. Basically it removes some of the erratic behavior of the bb. To test that compare a g3 to a Mp5k. The K will spray BBs everywhere wheres the G3 puts out a beam of bbs.

From what I have seen the single greatest effect on the FPS of your gun is the level of your technical expertise. You can take the best parts in the world, slap them together and still have a gun shooting stock velocities. You can also take a new M120 and get a gun shooting consistantly 410 to 420.

JayZ28
02-15-2005, 05:31 PM
Wolv-

I'm currently working on an AEG barrel length/cylinder volume vs fps/accuracy article and would like to hear any thoughts you may have on the matter as to testing methodologies etc. Ideally I would test with every barrel length and cylinder available, but that's probably not possible. I'm working with a few sponsors on procuring as many combinations as possible but would appreciate any input you may have.

-Jay

AustinWolv
02-15-2005, 05:50 PM
Jay, to clarify, the objectives are ultimately what?
1. Optimum muzzle velocity
2. Optimum accuracy at a set distance
3. To illustrate accuracy difference between different barrel lengths at a set distance
4. #1 and #2 coupled
5. To illustrate BB mass in relation to accuracy (previously done by ASP)
6. Others?

Once I understand the "problem" better, I'll offer what I can. :)

JayZ28
02-16-2005, 10:59 AM
Not really aiming to solve a specific problem as to solve "all" problems ;) The goal is to create a matrix of sorts which correlates barrel length and associated cylinder volume to performance characteristics such as accuracy, muzzle velocity, and rate of fire.

PS. your argument regarding BB weight is a very interesting one, and I'd like to reference it in the Mega-BB Review I'm currently working on.

-Jay

AustinWolv
02-17-2005, 09:16 AM
Roger that.

Quick thoughts:
- Run accuracy test at 20-25 yards if you have the room
- Muzzle velocity readings are average of 3 shots
- Perhaps use Ver3 mechbox since motor is attached; easy and quick to actively test without having to rebuild entire gun; AK would be great platform IMO for this as it easy to swap barrels and/or pull the mechbox. The pistol grip on the AK isn't necessary, but does help secure the mechbox, pulling it down so it sits correctly with the hopup/barrel assembly. Compare that to working with a Ver2 mechbox in which the pistol grip is a necessity. {shrug}
- Sounds like you'll have multiple matrices: ones giving groupings at each given BB weight, ones showing muzzle velocity at each given BB weight, and ones giving ROF at each given BB weight with the axes being barrel length and cylinder
- For muzzle velocity/energy, make sure hopup is off. The other two goals leave it on obviously.
- Obviously use same mechbox and all other same parts. Only parts that should swap between tests are barrel and/or cylinder. For example, cylinder Type0 is ran first......just swap out the barrel through the whole length lineup, running each with the different weight BBs. And so on....
- Consider running the accuracy test with 10 shots, and I'm confident that you were going to secure the gun bench-wise.
- Pick a well-regarded BB and use that for all the testing, if possible.
- Consider running a 100 rounds or so through each cylinder once installed, so that any inconsistencies due to lube are worked out. Clean the barrel after doing so of course.

Nothing ground-breaking there, but that's all I got for now. blake could probably offer some better insight if there is any.

JayZ28
02-17-2005, 03:09 PM
- Run accuracy test at 20-25 yards if you have the room

Check. I actually planned to do 25,50,75. Have ready access to very long indoor hallways.



- Muzzle velocity readings are average of 3 shots

Actually I've been averaging them over 10 shots for my other reviews, so I may just stick with that.



- Perhaps use Ver3 mechbox since motor is attached;

Very good point. I've been considering an AK or a G36 as a test platform exactly for the reasons you list. The fixed motor cage is also a plus. Not sure if it'll happen since the money is all going towards parts right now, but definitely considering this strongly.



- Sounds like you'll have multiple matrices: ones giving groupings at each given BB weight, ones showing muzzle velocity at each given BB weight, and ones giving ROF at each given BB weight with the axes being barrel length and cylinder

Yep. A multi-dimensional matrix probably wouldn't be very informative ;)



- For muzzle velocity/energy, make sure hopup is off. The other two goals leave it on obviously.

Check. Been leaving hopup off for all existing FPS tests.



- Obviously use same mechbox and all other same parts. Only parts that should swap between tests are barrel and/or cylinder. For example, cylinder Type0 is ran first......just swap out the barrel through the whole length lineup, running each with the different weight BBs. And so on....

Yep, that's precisely my plan.



- Consider running the accuracy test with 10 shots, and I'm confident that you were going to secure the gun bench-wise.

Yep, the gun will be secured on a workbench and aimed with a cheap laser attached to the gun. Levels will also be used to make sure the gun is... um... level (duh).




- Pick a well-regarded BB and use that for all the testing, if possible.

Planning to use whatever wins the BB Mega Review I'm currently working on. Previous tests have always used TM BBs, but that might change pending the results.



- Consider running a 100 rounds or so through each cylinder once installed, so that any inconsistencies due to lube are worked out. Clean the barrel after doing so of course.

Very good point. I'll definitely do this.


-Jay

joeyphoshowey
04-01-2009, 05:38 AM
Wow. Undertaking these procedures is an amazing time commitment, not to mention tedious.
Good luck with the tests.

It'll be a great reference. (understatement)

It's been four years since, how is the progress coming?
Abandoned?

AustinWolv
04-01-2009, 09:58 AM
Jay became very busy and never got to this project if I recall correctly.

The linked picture (http://www.lonestarairsoft.com/forums/photopost/showphoto.php?photo=6513&cat=500) courtesy of another forum might be useful however.