You won't find pre-chambered barrels ready to screw onto your FN. Not only is there setting of headspace to worry about, but also the timing/clocking of the extractor slot that is cut into the end of the tenon.
You'll need to buy a 6mm barrel blank and have a reputable gunsmith do the work.
Decide on a gunsmith first - he may have a suitable barrel blank in inventory. If not, bugholes.com and grizzly.com are two good resources to find in-stock ready to ship premium barrel blanks.
Good luck.
