Join Date: Oct 2004
Location: 1130241W5

Apparently "barrel suck" is a myth.
http://www.airsoftsociety.com/forums/f37/boreup2022/
Quote:
Barrel Suck, BS or Fact?
I’ve been looking at the whole Barrel Suck phenomenon and I just can not accept it as given.
I needed proof. So I decided to formulate some calculations as a starting point.
I’ll cover a lot more in this post, so please read through it fully before posting knee jerk reactions to my hypothesis.
For the purpose of this, I need to clarify some terms. Most will know them, but just in case…
Definitions;
aeg: automatic Electric Gun (airsoft)
Barrel Suck: The phenomenon that apparently occurs when an aeg piston begins retracting while a bbs.html'>bb is still in the barrel, causing a loss in FPS due to the piston sucking back air from the barrel behind the bbs.html'>bb.
FPS: Feet per Second – Velocity of the airsoft projectile
IPS: Inches per Second (FPS X 12) used for calculations within this hypothesis.
ROF: Cyclic rate of fire measured in Rounds per Second or RPS
My hypothesis:
Barrel Suck is a misnomer perpetrated on the Airsoft Community at large due in large part to certain active persons in forums across the world coining the buzz phrase with little more than speculation and guesswork and a great degree of “Technical Gobbs.html'>bbledegook” to fool the masses into a perpetual belief that what they say must be true.
Here are some mathematical statistics and formulas that can be duplicated to match any guns specific performance values. This is assuming the aeg is in prime operating condition and NOT double feeding due to magazine or hopup issues.
Test Bed #1 (Long barrel, Low FPS, High ROF):
Inner Barrel Length : 600mm (600mm X 0.03937 = 23.622 in, but let’s call it 24”)
FPS: 250 (about the lowest FPS for a decent gun)
Cyclic Rate: 20 RPS
Calculations:
At 250 FPS, it takes a bbs.html'>bb 0.008 seconds to travel 2 feet (24”, or just over 600mm).
[1 second, divided by 250 feet, times 2 feet]
At 20 RPS, a bbs.html'>bb is fired once every .05 seconds.
[1 second divided by 20]
Assuming Zero Point on our timeline to be the point where the piston releases from the sector gear, the bbs.html'>bb exits the barrel at approximately 0.008 seconds, leaving 0.042 seconds before the next shot even begins to travel down the barrel.
[0.05 minus 0.008]
The bbs.html'>bb is only in the barrel for the first 16% of the firing cycle.
Test Bed #2 (Long barrel, High FPS, Moderate ROF):
Inner Barrel Length : 600mm (600mm X 0.03937 = 23.622 in, but let’s call it 24”)
FPS: 400 (the golden number for many)
Cyclic Rate: 15 RPS
Calculations:
At 400 FPS, it takes a bbs.html'>bb 0.005 seconds to travel 2 feet (24”, or just over 600mm).
[1 second, divided by 400 feet, times 2 feet]
At 15 RPS, a bbs.html'>bb is fired once every .066 seconds.
[1 second divided by 15]
At Zero Point (piston release), the bbs.html'>bb exits the barrel at approximately 0.005 seconds, leaving 0.061 seconds before the next shot even begins to travel down the barrel.
[0.066 minus 0.005]
The bbs.html'>bb is only in the barrel for the first 7.5% of the firing cycle.
Test Bed #3 (Average barrel, Average FPS, Moderate ROF):
Inner Barrel Length : 363mm (363mm X 0.03937 = 14.29 in, but let’s call it 14.5”)
FPS: 350 (pretty standard for non TM stock guns)
Cyclic Rate: 15 RPS
Calculations:
At 350 FPS, it takes a bbs.html'>bb 0.0035 seconds to travel 14.5 inches.
[1 second, divided by 4200 IPS (350 FPS X 12), times 14.5]
At 15 RPS, a bbs.html'>bb is fired once every .066 seconds.
[1 second divided by 15]
At Zero Point (piston release), the bbs.html'>bb exits the barrel at approximately 0.0035 seconds, leaving 0.0625 seconds before the next shot even begins to travel down the barrel.
[0.066 minus 0.0035]
The bbs.html'>bb is only in the barrel for the first 5.3% of the firing cycle.
Based on these examples, anywhere from 84 to 94.7% of the time (while the gears are spinning) there is nothing going down the barrel!
Also busting this “Barrel Suck” myth is the actual function of the aeg gearbox..
Before the sector gear teeth can even engage the piston teeth, the tappet plate is disengaging the nozzle from the hopup chamber thanks to the ingenious location of the tappet nub.
While the piston is retracting, it will draw air through the nozzle, but since the nozzle is no longer sealed within the hopup chamber, it can not suck air from the barrel. Try sucking soda through a straw with the straw opening just millimeters outside of the liquid. All you’ll get is air. Your sucking has no affect on the soda, just like the piston has no affect on the bbs.html'>bb in the barrel (if it’s even still IN the barrel).
Now, with all of that being said, there is a problem which occurs with improper cylinder sizing which will affect FPS, Accuracy, Consistency, and Performance.
This problem occurs when your cylinder does not physically have enough air volume to fully propel a projectile down the entire length of the barrel.
Typically, cylinders have venting holes at certain positions along the length of the body. This is primarily to minimize air resistance during piston acceleration after release in small to medium barrel lengths. There are many resources available to guide you to the proper cylinder type for your barrel length.
Putting a 500mm long barrel in a gun with a cylinder designed for a 363mm barrel is looking for trouble. Your cylinder will not have the sufficient air volume to propel the projectile down the entire length of the barrel.
Use the following formula:
Cylinder ID in mm X Cylinder ID in mm X 2.1415 (Pi) X Cylinder length in mm (to hole, if it has one) = Cylinder Volume in Cu mm
Barrel ID in mm (if you don’t know the ID, use 6.08) X Barrel ID in mm X 2.1415 (Pi) X Barrel Length in mm = Barrel Volume in Cu mm
If your Cylinder Volume exceeds your Barrel Volume, you’re good to go.
If it’s close, you may want to move to a Zero or Bore Up cylinder.
So, in closing, the phenomenon known as “Barrel Suck” has absolutely nothing to do with barrel suck. It is simply a result of improper sizing of cylinder type to barrel length.

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