Note that I didn't say that the standard groove diameter for .50 caliber barrels is .512": the CIP standard for the .500 Nitro is established at .512", and that is the industry standard FOR THIS CARTRIDGE. The usual standard for current cal. .50 barrels is .510" groove diameter, and, as a custom barrel maker, if asked to make a '.50 caliber' barrel, unless the customer advised that it was for some specific cartridge or requirement which called for another groove diameter, that is what I would make - in fact, if I made a pair of barrels for a .500 Nitro to furnish to, say, Searcy for use in building a rifle for myself, the groove diameter would be .510". But, as a maker of true custom barrels, all parameters are variable to the user's requirement (within the mechanical limits of my equipment, and what I consider safe, or good practice).
I think you might be surprised at the actual burning temperatures of ball powders in high-pressure loads: they are heavily deterrent coated, but are typically double-based, and, if there is any real difference in the erosive effect of ball powders versus stick powders, in ballistically equivalent loads, I've never seen any evidence of it - and I've spent nearly as much time peering at the ends and insides of rifle barrels as a gyneco... errr... proctologist does in examining the orifices and, umm, plumbing which constitute his area of expertise.