I've been using that exact setup for over a year now. I use the Larue Tactical QD mount. Works awesome and I like it a lot. Holds AR hands free at any angle until you are ready to shoot. The trigger pistol grip on the Manfrooto makes running shots fast and stable. You can buy a thread reducer to fit the tripod screw on ebay or at a good camera shop.
http://www.ebay.com/itm/1-4-Female-to-3-...=item4d38a841b2
http://www.larue.com/manfrotto-322rc2-ballhead-lt666-adp-combo
this guy demos it
[MEDIA=youtube]7FIA9nb_SK8[/MEDIA]
I leave my Larue mount locked into Manfro though. Then drop my AR rail into mount when ready to use just as fast that way instead of leaving mount attached to rail all the time.