8x57R was never loaded with black powder, far as I know. It first came out with a 224 or 227gr. .318" bullet (didn't it) loaded with smokeless powder for 2,000fps. That case could not get that velocity with BP, impossible, even it if was loaded as a solid plug of it as with the earliest .303's. It will be a "J" bore, of course, as 88 noted. To do that, would need load the plug before the shoulder of the case was formed, as with the .303.
: BP by itself, if stored below about 35% humidity, will not rust steel. It is the additon of water or moisture that then starts oxidation, ie: barrel worms, ie: rust.
: On the other hand, Pyrodex fouling is corrosive in and by itself. I don't know about the fouling of Triple (777) noted above, but would not trust it. Too - 777 has been know to jump pressures around a bit, but with very light loads, ie: less than 3 drams, it should be OK - I just wouldn't use it myself - nothing but real BP for a BP proofed gun - if it is.
: I don't believe that gun is proofed for BP, but then, I don't have it to examine.