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Originally Posted by mcguyver
If you use a "windowless" mechbox in a gun with a full or over-capacity cylinder (no holes in it), there is a good chance your FPS will go right into the toilet. This is because on the "intake" stroke of the piston, the inrush of air in to cylinder is impeded by the lack of holes in the mechbox to allow for this volume to enter the mechbox and be drawn into the cylinder.
I did an ICS MP5 with full capacity cylinder and M4 length inner barrel. With a 110% spring, the FPS was about 240. I changed the cylinder to a 4/5 M4 cylinder from a Marui and the FPS jumped back up to 335. ICS uses a "windowless" mechbox as well.
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I think you have a more major issue at hand like a sealing issue with the nozzle and hop up. You also are not properly matched in barrel and cylinder volume when you use a full cyl and a M4 barrel.
With an unported cylinder and piston head, the only inlet for air is through the centre port in the cylinder head. You don't get any air filling the cylinder from the inside of the mechbox. If you want to do some hypothesis testing, try rotating your M4 cylinder so the side port opens towards the inside of the mechbox. If you're right about mechbox pneumatic issues, you'll observe the same chrony results as your full cyl' results.
With ported piston heads, some air from the inside of the mechbox enters the cylinder, but the rearwards motion of the piston would serve to compress the air in a sealed mechbox which would push more air into the cylinder. However mechboxes are not sealed well. There are several openings which are larger than the combined area of the openings in a ported cylinder so they serve a very small pneumatic resistance compared to the pneumatic resistance that the holes in a piston head present.
On the way forward, accelerating the pellet down the barrel presents a significant restriction. Also, the size of the cylinder head port is quite small compared to the combined area of various openings in an unwindowed mechbox so even if the bb wasn't the dominant resistance, the hole in the cylinder would be.