It depends on the maltster. Typically, black roasted barley is around 500L, while I believe English Roasted is a bit higher near 600L. There are most likely subtle taste differences too.
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It is within the imagination to presume a black roasted barley could hit 600L and an english roasted barley could be down around 500L. These things aren't exact and swapping one out for the other shouldn't hurt anything, particularly if you are using them in a small quantity, which I imagine you are.