The "Bull" gun was never used by the military as a sniper rifle. The Model 70 sniper rifles were built with a Douglas barrel from standard Model 70 rifles. I'm not sure any of the rifles had factory Winchester barrels for Vietnam.
Here's a good read:
http://www.americanrifleman.org/articles/military-winchester-model-70/And if you are interested in building one, this guys have the correct barrels for you:
http://www.texasbrigadearmory.com/m70.htm