Just curious, but why do you want a 5mm over a standard 22 mag, 17 HMR or maybe even a 22 hornet? I've read a little on the 5mm but with ammo only available here and there I was wondering what real advantages that particular cartridge would give you.
I've shot chucks with a 17 HMR and could not believe how effective it was on the little creatures. I have since bought a 22 mag barrel and can't wait to shoot one with it to see how it compares, but I have to confess that the archery bug has bitten me and sniping the little bounders with a bow is urban hunting at it's finest!
I've always wanted to meet someone that has a 5mm, just cause it would be cool to shoot and maybe that's what you want. A good chuck is a dead chuck! Good luck finding what you're looking for.