ICS FTK Upper Mech box difference
 

I'm interested in these two kit from ICS Airsoft: MA-54 and MA 56.

The MA-54 comes with MC-71 OVERSIZE CYLINDER (FULL CAPACITY)

And

The MA 56 comes with MC-58 Oversize cylinder (4 opening)

How does the opening make a difference in the performance of the gun?

My plan is to get the FTK, and just keep my stock spring. Im looking for better seals, and compression. I don't need the 380+ FPS.

Do you have an M4 of M16?

4 openings is for M4 (shorter barrel) and no opening is for M16 (longer barrel).