Quote:
Originally Posted by bissa
is it possible that the cylender was too small and you were getting a minor amount of suction with the longer barrel, hence when you put in a shorter barrel the suction went away and your FPS improved?
i am new to this, but it is just a thought.
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There is no such thing as "barrel suck".
First, if you have a bellows piston head, it sucks minimally on the back stroke.
Secondly, the air nozzle is disengaged from the hop up chamber by the tappet plate before the piston is pulled back, breaking the air seal, so if there was any "suck" it would come from around the hop up chamber.
Thirdly, if the retracting piston did suck from the hop up chamber, it would suck the next BB being fed up from the mag, not the BB in the barrel.
To see how a mechbox works, see:
http://www.mechbox.com/
Further more, let's take the following example, an M16 shooting at 300fps with a .20g BB at 20bps. The velocity is on the low end of AEGs and the rate of fire is on the high end.
20bbs/1second = 1bb/.05seconds (one BB fires every .05 seconds).
300feet/1second = 15feet/.05second (the BBs travel 15 feet in .05seconds)
15feet X 304.8 = 4572milimeters (convert feet into milimeters)
That means the first BB is 15feet or 4572milimeters downrange when the next BB is fired.
4572milimeters/.05second = 509milimeters/.006second (it takes .006seconds for a BB to clear a 509milimeter barrel)
That is 8.3 times faster than the BBs fire. In other words, the BBs clear the barrel in 1/8 of a firing cycle.
A waveform picture an AEG firing cycle:
http://team-titanium.com/~airsoft/AE...tricReport.pdf
Even if the piston head did fully suck while winding and the air nozzle remained tightly sealed to the hop up chamber, and did not suck the next BB being fed, the BB is still way downrange before the piston begins it's next cycle.
Furthermore, a typical AEG setup has the air volume of the hop/up barrel system to be about 58% (more or less) of the air volume of the cylinder. This has been found to provide the most efficient setup for muzzle velocity and accuracy. If the volume of air in the barrel system is too great (long barrel with a ported cylinder) the air coming from the cylinder will stop pushing on the BB before it exits the barrel. This causes a net loss of muzzle velocity and unstable BB flight characteristics. This is sometimes also known as "BB suck", though there is no actual suction, just a loss of forward pressure.
Conversely, for shorter barrels, it is more efficient to "port" the cylinder, allowing the piston to accelerate for a distance before it begins to push air down the barrel. If you just push a full cylinder (type 0) volume of air down a short barrel, the BB will leave the barrel before the column of moving air reaches it's maximum acceleration. A ported cylinder (type 1, etc) uses the wasted air space to allow the piston to first get up some speed before pushing air down the barrel. This allows the maximum transfer of energy from the piston to the air in the cylinder/barrel.
Ported cylinder setups are especially useful with weaker springs where the initial back pressure in the cylinder/barrel system could cause a net loss of muzzle velocity. Stronger springs and piston weights change the dynamics involved in cylinder type/barrel length setups.