The only Tac-50 I know of is made by McMillan
As for Accuracy International the only 50 I thought they made was the AW-50
And yes cow is right it is a TRG-42, it is not a .50 cal ether its a .300 Winchester
Here is more info on that rifle:
http://en.wikipedia.org/wiki/Sako_TRG
Here is a list of all the .50 cals I know of:
* Accuracy International AW50
* Accuracy International AS50
* OM 50 Nemesis
* Anzio Iron Works Anzio-50
* Armalite AR-50
* Pindad SPR-2 and Pindad SPR-3
* Barrett M82/M107
* Barrett M95
* Barrett M99
* Bluegrass Armory Viper
* ČZW-127
* DSR-50
* East Ridge / State Arms Gun Co. Inc.
* Bohica MK III AR-15 Upper
* Bushmaster BA50 Rifle
* L.A.R. Manufacturing, Inc. Grizzly Big Boar
* Halo Arms, LLC HA-50, HA-50 FTR
* McMillan Tac-50
* POLY-Technologies M99-II and M99B-II
* Ramo M600 and M650
* Robar RC-50
* Safety Harbor Firearms SHF/R50
* Serbu Firearms BFG-50
* Steyr HS .50
* TGR Co. Noreen 50 BMG
* Gepard anti-materiel rifle
* Zastava M93 Black Arrow
* PGM Hecate II
* Ultralite50/Ligamec Corp.
* Vulcan Armament V50
* EDM Arms Windrunner
* WKW Wilk
* Zel Custom Manufacturing/Tactilite