My P5 when new would do up to 565fps or so with Gamo Hunters(7.5ish gr?)about the lightest ammo I have/had on hand. I've got alot of rounds through it now and also removed the spacing from the piston and it does a much more comfortable 485fps with JSB Schaks(8.2gr). After figuring out hold it's a pretty accurate pistol, with or without the spacer.
You can get the .22 P5 Magnum here;
http://www.dlairgun.com/home.htm
HTH,
Mulby