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Old September 15th, 2011, 16:56   #28
GBB Whisperer
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Join Date: Jun 2002
Location: Toronto
Originally Posted by ThunderCactus View Post
Most important thing is not necessarily barrel diameter, but barrel QUALITY. I've personally seen high quality 6.08 barrels outperform cheaper 6.05 barrels. As long as your BB doesn't hit anything, get redirected, and you maintain laminar airflow behind the BB, you get stupid crazy accuracy.

One thing that came up lately though is initial pressure.
And extremely important concept when comparing GBB to AEG.
For example, most of us use heavy ammo in pistols, even though they're only 280-300fps. You'll put a .25g or .28g BB into a STOCK 300fps pistol and get amazing range and accuracy out it.
Whereas if you put a .28g BB into a stock marui, even with optimal hop, your BB falls far short of a .20g!

So correct me if I'm wrong, but what I've figured is happening, is the AEG doesn't have enough initial pressure to get the spinning velocity it needs from the hop rubber. Because the piston starts it's travel so slowly, although there IS pressure being created, the BB may only be doing say 100fps when it leaves the hop rubber, and accelerates as it goes down the barrel. Of course the number goes down if you don't have a ported cylinder, since the piston doesn't have the velocity to build high pressure quickly
Whereas in a GBB, the propellant you are using is already compressed, and only needs to be applied. So right off the bat you start of with WAY higher pressure behind the BB. So when it leaves the hop rubber, let's say it's doing 200fps. So you get way more backspin on the BB, and with enough gas left to continue accelerating it on it's way out the barrel.

So my theory is based on the field performance of stock GBB pistols using heavy ammo, and both stock and upgraded AEGs of the same velocity using the same ammo. Infact sometimes of higher velocity, my 320fps HFC beretta was on par for range with some pretty nice 380fps AEGs...
Also, when you adjust your hop rubber, it's not so much the surface area of the rubber that matters, but the PRESSURE IT APPLIES. Making the gun require more pressure in order to get the BB past the hop rubber. And more initial pressure behind the BB means more inital FPS.

So if you compare a stock GBBR with a final muzzle velocity of 380fps on.20g BBs, with a stock AEG of the same. The GBBR should get better performance, because the GBBR can more effectively use heavier ammo, due to it's increased initial velocity.
Now comparing fully upgraded and tuned AEG's and PTWs (is it bad form to group those together? LOL), the gap isn't as far.
The only problem that occurs is in cooldown and ambient temperature, where your gas pressure drops (or raises) significantly, so it's going to have less short term repeatability over an AEG.
So like any other airsoft science, you can't use just one aspect to pick a gun. The GBBR will perform better stock, but it's a balance between how you shoot and the repeatability of the gun.

Ugh that's a mouthful
What do you guys think?
I believe the theory is something completely different, and as follows:

Newton's Laws of Motion

The basis of classical mechanics. Think of it this way, it requires less energy to push an empty shopping cart, than a fully loaded one. Once you have it pushing, it will leave your hands rolling at greater energy than the empty cart, even if they are traveling at the same speed.

The reason why you see GBB's exhibiting a performance increase when switching to heavier ammunition, has nothing to do with initial pressure. Rather, it has everything to do with the expansion co-efficient of the compressed gas, combined with the liquid to gas phase conversion, AND the kinetic energy required to move a heavier mass versus a lighter mass.

The first thing that needs to be understood, is that spring powered pneumatic compression will always render the same amount of expansion energy from the spring. The spring will not push harder if there's a heavier projectile. The spring will not push less if there's a lighter projectile. If the spring is rated to push 1 Joule, it will push 1 Joule. End of story. This is just the basic nature of spring power.

Thus, SPRING POWER IS LINEAR, regardless of the mass of the projectile.

e.g. if you fire a 0.20g BB out of a spring powered gun at 1 Joule, it will deliver ~328fps at the muzzle (in theory. This is excluding other factors that affect power output.)
If you fire a 0.25g BB with the exact same gun set up with the 1 Joule spring, it will fire at 294fps. Despite being a lower velocity, the power relationship is linear, and 294fps with a 0.25g BB is STILL 1 Joule of power.

HOWEVER, with gas powered guns, the power output is NOT linear. This holds true, whether we are talking about a gas blowback airsoft pistol or a real steel sniper rifle. The concept of expansion co-efficient remains the same.

When you use a heavier projectile in a gas system, the muzzle energy will become HIGHER than if you were to use a lighter projectile.

e.g. if you fire a 0.20g BB from a gas gun rated at 1 Joule, it will deliver ~328fps at the muzzle (in theory. This is excluding other factors that affect power output.)
However, if you fire a 0.25g BB with the exact same gun set up, using the same gas, the BB will fire at a HIGHER muzzle energy. The actual velocity and muzzle energy delivered will depend on many factors, including the length of barrel, the bore, the gas routing system, the gas used, etc. The largest contributing factor is barrel length and bore. The longer and tighter the barrel, the larger the power curve increase.


It is NOT uncommon to see a gas gun shoot a 0.20g BB at 1 Joule (328fps), but once you drop in a 0.28g projectile in there, it will all of a sudden punch it out at 1.6 Joules, equivalent to 335fps! This is an extreme example, but it's not impossible for it to happen. On lower ends of the scale, the difference is not as extreme. In a Tokyo Marui Hi-Capa, for example, it is not strange to see a 0.1 Joule increase by going from 0.20g BB's to 0.25g BB.

In layman's terms, this means that shooting a 0.25g BB from a gas gun is going to provide more point blank pain than a 0.20g BB. The heavier the projectile shot out of a gas powered gun, the more it will hurt at the muzzle.

The Theory Behind the Phenomenom
Compressed gas requires time to expand, particularly in phase change systems like the ones we use (it converts from liquid to gas.) The volume of propane in gaseous state is higher than the volume of propane in a liquid state. The more time the gas has to expand, the greater the expanded volume.

When you fire a heavier BB from a gas gun, it stays in the barrel longer. It takes MORE energy to start it moving. It takes MORE energy to accelerate it. It takes MORE energy to push it out of the muzzle. The longer it takes to push it, accelerate it and move it, the more time the gas has to expand!

So, the basic differences between spring power and gas power, is that spring power output is a linear rate. A spring can't push MORE power if it needs to. Springs just don't work that way, whereas gas power output is at a curved rate. A gas system will use up as much gas and power as is required to push the mass and accelerate it to speed.

As an addendum on the mechanics of airsoft guns:

Spring expansion stops when it reaches it's terminal (physical) limitation. In the case of an AEG, this is when the piston head smacks the cylinder head. That's the end of the line. In some cases, the BB may not have even left the barrel yet!

Gas guns, on the other hand, generally work on a completely different mechanical principle. Gas guns work very much like a real gun in terms of the projectile release and mechanical cycling of the next round. In gas airsoft guns, as well as real guns, the slide or bolt does not start moving until the projectile has long left the barrel. Even when the projectile has left the barrel, the valve knocker is STILL holding the gas release valve open, and gas continues to pour out. Once the projectile has left the barrel, the slide or bolt begins the rearward cycle of the gas expansion process, at which point a disconnect lever is triggered to release the valve knocker. Until this valve knocker releases its power, it will continue to eject gas in to the chamber. Thus, you will never see "suckback" issues that you'll see in spring systems. The system is designed to use as much gas as is required to clear the BB from the barrel, regardless of the weight of the BB or the length/bore of the barrel. (This applies to most modern airsoft gas guns. Old gas guns had a set amount of gas output, regardless of projectile timing.)

"What does the muzzle power mean?"

The muzzle power refers to the energy that the projectile is leaving the muzzle. In layman's terms, 1 Joule of power at the muzzle will give you the same amount of point blank pain, regardless of how heavy the BB is. Thus, 1 0.20g BB at 328fps will hurt JUST AS MUCH as a 0.25g BB at 294fps. It is still 1 Joule of power.

UNDERSTAND, that fps is NOT a true indication of power. Muzzle Energy (measured as "Joules") is the true indicator. As an extreme example, let's just say that I would rather have a 0.01 gram feather smack me in the face at 500fps, than a 20 tonne boulder smack me in the face at the same velocity. One of those items will cave my face in. Guess which one it will be? The heavier item will carry a much larger energy than the lighter one.

Why Understanding this is Important for Airsofters

It's simple. Safety reasons. It's the reason why I continue to urge game hosts to set MUZZLE ENERGY limitations with the intended projectile weight, rather than to set velocity limitations!

If I were to be sneaky, I could step in to a game rated for 400fps max with 0.20g BBs (1.48 Joules) with my gas gun set up for that power, and it would pass. The game hosts have set up the game this way, thinking that at 400fps, it will minimize safety issues. However, once I hit the field, I could drop in 0.36g heavy sniper rounds, and all of a sudden, I could easily be punching out 1.8 Joules because of the above mentioned gas dynamics. Now, this 1.8 Joules of power is equivalent to 441fps with 0.20g BB's. In some cases the difference could be even higher! All of a sudden, I just gained 10% in power, I'd cause 10% more pain/damage, and likewise, I SHOULD be aware of this so that I would increase my minimum engagement distance.

Again, the increase is dependent on many factors, but this is to outline the safety concerns.

That was my brain barf.\

Last edited by ILLusion; September 15th, 2011 at 17:16..
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