Browning BAR Safari comes in 338 Win. Mag. It is not stainless or synthetic though.

http://browning.com/products/catalog...31&type_id=001

Just wondering why you want or need a .338 in semi auto? That' s usually not a cartridge chambered in a semi auto rifle. This is just my opinion but I think a bolt action would serve you much better than an auto in a caliber like a 338.