[Styrak]

A P90 is not supposed to have an unported cylinder since it's such a short barrel.

With thje shorter barrel it's having very "ineffiecient air usage" I would call it. The BB is leaving the barrel before the piston completes it's cycle and there is wasted air expelled after the BB leaves the barrel and then the piston slams into the front of the mechbox with no BB in the barrel as resistance. That explains the loud sound with the short barrel.

The longer barrel fixes that, as an AK should have an unported cylinder. The BB stays in the barrel longer and get more energy transferred to it, hence a higher FPS.

I don't think it's necessary, might save some wear and tear on your mecvhbox (V6 are pretty beefy anyways) but if you want you can choose which barrel you want to use, and if you want to use the short barrel get a gundoc to switch out your cylinder.