What do you lose or gain by going with a shorter barrel?
You lose six inches, and I want to say 200fps on the ammo (actually a good thing, as the bullets don't hold together quite as well at the higher speed, having been designed for optimum fragmentation/tumbling at P90 speeds). And the gun also looks a heck of a lot better; the PS90 looks & feels like it's half barrel. You do get a little more juice into the bullet from the extra length, but it's well into the diminishing returns region (the cartridge was designed around a 10" barrel and does not benefit from additional bore like 5.56, for instance)
If I'm looking at the barrel right, there is a shroud that does not look like it should be threaded.
FN Herstal not only made the gun 16" barrel length, they made it a royal pain to shorten it; they went to such lengths, I have to wonder if it was not done to satisfy either domestic or foreign export civilian sales restrictions (sort of like how semi-autos cannot be readily convertible to fully automatic).
-There is a shroud over the barrel which forms the flash hider. It is threaded on, but also pinned,
with a 3/4" long pin hammered in blind around the circumference of the barrel threads. Very, very difficult to cut that pin out and not destroy the shroud or damage the barrel threads. I believe the threads are lok-tited, too.
-At that point, with the shroud off, new threads will be required to be cut into the barrel for a muzzle device. However, since the P/S90 action requires the barrel to slide forward/backward about 1/8" against a return spring as part of the extraction timing, the exact placement of the muzzle device and its shape are kind of important.
-Complicating muzzle device installation, is that the PS90 barrel has a removable barrel nut at the chamber end (necessary because the flash hider/shroud is permanently attached), whereas the P90 simply has wrench flats (and a threaded muzzle). This means that care must be taken while torquing down the flash hider or barrel nut so the other does not come loose. I believe the PS90 nut requires a special narrow wrench, too, or a modified box wrench.
So although it can certainly be done, cutting down a PS90 barrel involves cutting/damaging an expensive FNH barrel (~500$) and shroud, and still requires some careful gunsmithing on top of that. Which is why most folks just buy a reproduction P90 barrel, or an original if they have the coin and really care about chrome lining. I personally found even removing the PS90 barrel to be too much work, which made the EFFEN90 replacement receiver even more attractive (now I'll have two 'uppers' I can swap out of my stock at will with a push of the takedown button)
TCB