CA M15A4 low FPS (airloss problem?)

[Flatlander]

Baught an almost new, stock CA M15A4 RIS this summer. Played with it for a couple games before I had a PDI 150% spring put in with a tightbore. Before upgrading it was shooting about an unusual 170 FPS and after upgrading the spring and tightbore it maxed out at 300 FPS. I thought something was wrong as I was expecting somewhere around 350 FPS from reading around the forums.

Several games down the road after the upgrades I figured that something must be leaking like a SOB so I taped up the port hole on the cyclinder. After taping up the cylinder port I noticed it had quite a bit more zip (no chrony confirmation but it was an obvious FPS increase).

With the winter here I figured I'd get this bugger sorted out, hopefully. Anyways, I plan to tinker with it myself and get my hands dirty so any suggestions as to what may be wrong and what I should be looking for? Seems to be an airloss issue. I remember reading somewhere that the new CA's were coming out with very low FPS - no explanation why though.

Thanks.

[Crunchmeister]

New CA M15A4s advertise 280-320 FPS out of the box. I'm not sure why this large discrepency on velocity though. With my M15 rifle, I do notice that some shots on semi auto seem to be less powerful, but they all hit hard in full auto. I have no access to a chronograph, but I'd say the shots are all at least as powerful as my stock TM M4 in semi auto, and even harder in full auto.

But it sounds like you may be right. You may have an air leak in your piston or cylinder. Perhaps a replacement cylinder and piston would help. But take this suggestion. I'm just a noob, so I'm throwing out a guess here.