Can't say that I've ever seen a coating like that...I would be curious to know just what it is.
I'd highly recommend a gas checked bullet of 350-400 grains....specifically either of Ranch Dogs designs (350, 425) or an RCBS 400 grainer.
These work well "as cast" in my guide guns as well as Sharps and rollers, and leave a pristine barrel, absolutely clean with Reloader 7.
I run these through a .460 sizer only enough to seat the gas check
The gas checks are well worth the extra expense, eliminating most fliers, and scraping the barrel clean with each pass.