i read somewhere that the 2009 and newer Gemtech Halo silencers dont work with the FSC556...is this true?

I beleive that is the case, though I cannot remember where I read that.

PWS website states this:
http://www.primaryweapons.com/STORE/PC/viewPrd.asp?idcategory=&idproduct=9

Check the suppressor section, and Gemtech's Industry section, there were several threads about the change to the new 2009 Halo.

The older style worked with the FSC and the A2, while the new version works with the A2 and the Smith Ent. Vortex. Apparently, in order to get the Vortex to fit, they had to change some of the internal geometry of the mounting system, which may have caused the FSC to no longer be compatible. It seems like a good move, the Vortex is an outstanding FH and very popular.

I wonder why PWS would make some new FSCs to work with the Gemtech G5 when Gemtech would make a bonehead change like that to the Halo. IMHO the FSC is better than the phantom, but that's just me....