Welcome to the forum. Interesting first post.
A .50 BMG case is .804" in diameter at the largest point. That is close to a 9* gauge (.803"). See this chart
http://en.wikipedia.org/wiki/Gauge_(bore_diameter" onclick="window.open(this.href);return false;) Don't know anyone that has a 9 gauge and as already mentioned the case is rimless, so would not be something you would be able to use. Besides, I would not want to waste brass from a .50 BMG and try reworking it when you can buy plastic 12 gauge primed hulls for as low as $11 - $12 per 100. Even brass hulls are only a little over $1 each if you want something for old black powder loads and want to stay realistic.
