[MadMax]

if the volume of the cyl is too small compared to the volume of the barrel, your mechbox may not push enough air to keep the pressure high by the time the pellet reaches the end of the barrel.

Imagine a hydraulic AEG which pushes a syringe filled with water into a barrel with a bb leading the water. Assume no leakage around the bb. If the syringe had a smaller volume compared to the barrel it wouldn't push enough water to push the pellet to the end of the barrel.

The situation is kind of similar in an AEG except that air is compressible so you need an even larger syringe volume because the system operates at pressure (the air initially starts at 0psig). Not having enough volume would have the pellet drawing the pressure in the cylinder and barrel below atmospheric pressure for the last bit in it's travel so you'd actually see muzzle velocities drop if you increase barrel length past a certain point.

I believe this is the proper description of the term "barrel suck".

A couple years ago I compared the ratio of cylinder volume (volume ahead of the port) vs. barrel volume and noted that matched cyl-barrel pairs had the same volume ratio for each TM AEG so I strongly suspect that good old Marui had done some design work of their own to determine an optimal ratio. I didn't end up confirming it myself as I had other stuff to do, but someone could test an AEG by starting with say a M4 cyl and keep trying longer and longer barrels starting from an MP5k up to a 650mm PSG barrel while measuring fps.