I don't believe suckback exists.
Matching barrel/cylinder size (or hpa pressure or stuff like that) IS important, but not for the reason of eliminating 'suckback'.
From what I can see, the definition of suckback:
Suckback occurs if the BB is still in the barrel when the piston begins it's backward movement, thereby creating a vacuum and in effect 'pulling' the bb backwards, destroying fps and accuracy.
My reasons that this ISN'T the case:
When the piston is moving backwards: The nozzle is disengaged from the hopup unit, also, the piston doesn't seal as well on the way backwards. Because of this, the 'maximum' suckback you could get is equalizing the barrel behind the bb to near atmospheric pressure.
From the time the piston head moves past whatever vents you have in your cylinder, or just starts moving forward, the bb begins accelerating. Say you have a modest gun, about 300 fps. Even taking into account that it takes time to accelerate, the average velocity is ~45.72 m/s, assuming linear acceleration. Even with a long barrel (550mm), it takes just .01 seconds to leave the barrel at this speed. Assuming your gun has a moderate rof, say, 20rps, that's only 1/5 of the cycle of the gearbox. The piston stays forward for approximately 1/2 - 2/3 of the cycle (Look at the sector gear) of the gearbox. Since your bb has left the barrel before your piston begins moving back, whether or not the nozzle has disengaged, which it has.
There is a chance with high-fps guns, with very long barrels, that your bb will leave the barrel before your nozzle has disconnected. This is not the 'fps drop' usually assosciated with "suckback". This CAN somewhat emulate the results of the standard 'suckback' explanation by creating a small vacuum pressure behind the bb if too small of a cylinder is used... simply because nothing behind the bb is venting before it leaves the barrel. This is mathematically a very small vacuum, if any. Your bb still isn't being 'sucked back by the rearward movement of the piston". This is one of the closest ideas to suckback that I feel makes any sense. Even with this, it won't hurt your accuracy, because it will do the same thing every cycle, and doesn't create turbulent airflow, it will just drop your fps slightly.
The reason for ported cylinders:
Blowby, a very noticable problem, occurs when you use too big of a cylinder. You put too much air behind the bb, and it rushes out around the bb when it leaves the barrel. As it does this, it's extremely turbulent and will seriously hinder your accuracy.
The reason too small of a cylinder hurts your accuracy AND fps:
Barrels, they aren't perfect. BBs, neither are they.
When you lose pressure behind your bb (whether it be to the small amount of vacuum, or a vent to atmospheric), it loses the cushion of air that is trying to keep it centered in the barrel, and begins to roll along the bottom surface. Because of this rolling, you have extra friction through the barrel (reducing fps), and your hopup is being counteracted by this friction along the bottom of the bb(Reduces velocity further by requiring you to turn your hopup on higher). If your bb's and barrel aren't perfect, then you get inconsistent 'rolling' effects, greatly impacting your accuracy/precision.
Logic for those who think suckback exists:
If I use a cylinder for a 200mm barrel length on a 500mm barrel, I will get some major suckback, right?
In fact, I should get the same amount of suckback (200mm's worth) as I originally put into the bb. So the bb should just STOP in the barrel, right? Since the same amount of energy that was put into the bb was taken out of it, it should come to a complete stop at 400mm in the barrel. Why doesn't this occur if suckback exists?
