Technically this should be legal here, the problem is , it is likely a US made product, hence illegal to export from the US due to its caliber.
Personally I fail to see the point of a 50 BMG handgun, how much powder needs to be wasted to get a bullet a couple hundred yards downrange. 308 is gross overkill in a handgun. In a rifle where you can get 2 miles of useful range I understand and thrive on