The Five-seveN forums has a good forum on reloading. Note: the 5.7x28 cases are covered in a polymer/lacquer coating (almost the same as the Wolf steel cased ammo). We have long speculated on the exact purpose of this coating, and came up with a few theories:
1). For proper feeding and functioning in the plastic 50rd P90 magazines.
2). For proper extraction when fired out of the P90/PS90. The chamber pressure is rated at 50,000 psi and it is a direct blow-by system, so it's trying to yank out the cartridge with a lot of force being exerted on the chamber walls by the case. FN designs the barrel to recoil back a little (Remtek states 0.030", you can use a feeler gauge to verify the space between the front barrel sleeve and the receiver) to allow extra dwell time. So the lacquer/polymer coating probably liquifies slightly under load and allows easy extraction.
Also keep in mind that many of the 5.7 reloaders are shooting them out of the Five-seveN pistol, which was designed after the P90 and doesn't quite need the lacquer coating for the magazines and it probably has a different action system as well (browning or recoil action).
Re-sizing the cases usually strips off all the lacquer coating, but YMMV.
