I have a queston .... i am looking for way to mount my  fenix pd30 light to my  rail of my ar.... . what mount would work?  thanks

You'll want to ask this in the lights forum - a few links up from the optics forum.

The PD30's a tricky light to mount, due to the 0.85" barrel diameter.  It's too small for the standard 1" mounts, and too slightly big for the E series mounts I have (though some may fit).  It's also not an optimal tactical or weapon light, due to the multi-click user interface and the lack of a soft-press capability.  It's a great disaster/car kit light, if a bit big for EDC, IMO.

That said, I put a P2D (older 1 cell version of the PD30) on a shotgun to test it out, using the TRGear adjustable size mount (solid but big and awkward).
http://www.trgear.com/osc/eshop/product_info.php/products_id/212

I wanted to see both if the light would hold up, and if the modes would switch under recoil. It survived about a hundred rounds of 12 gauge buck and bird admirably, including when the mount flew off and landed on the concrete behind me, light still on.