The PS90 has an 8-groove, 1:7 right-handed twist barrel with 14.8" of rifling. By ATF measurement standards (because the measurement is from the front end to the bolt face, which thus includes the chamber length), this would be considered just over a 16" barrel because you'd add a bit over the 1.14" of case length to the overall length. This is considerably longer than the P90's 10.4" overall-length barrel. While information is unavailable on the rifled length of this barrel, the military barrel is around 4" shorter than the civilian barrel. This translates to the bullet spending more time in the PS90's barrel, thus developing slightly more power than the P90, from the exact same round.
Note: CMMG sells SBR barrels, available in both 1:7 and 1:9 twist rates. These are aftermarket replacements for the SBR P90, or for PS90s which have had the SBR paperwork run on them.
The longer barrel and longer rifled length of the PS90 will result in greater muzzle velocity and a higher spin rate than the shorter P90 barrel. This thus allows greater effective range or the use of heavier bullets than in the P90.
The P90's trigger is fully-automatic, and the PS90's trigger is semi-automatic. On the P90, pulling the trigger results in one shot, and pulling it in all the way will result in fully automatic fire.
Replacement barrels for the PS90 are not readily available. In order to address ATF's sporting purpose requirements, FN took some fairly involved steps to ensure that the barrel shroud and barrel were not readily removable. It's worth noting that the flash hider on the front of the PS90 is very long. A full-length barrel would add several inches of rifling, which would add muzzle velocity, spin, and overall firepower without deviating any farther from the P90's design than the PS90 is already doing in the first place.
-- SeanNewton - 20 Aug 2008