In the article black powder was stated as used for "Provisional Proof". This I think is partionally due to considerations of ignition as the bbls are not yet chambered but fired from a priming chain of powder. Also I seem to recall reading that a heavy charge of coarse grained BP gave a better pressure curve for testing the entire bbl than was possible with smokeless. for a number of years I think definitive proof was done with a finer grained BP, but seem to think that smokeless is now used instead.
It is also noted that according to Winchester when they developed their Glass Fiber wound bbl with steel liner it withstood being fired with a 20ga ahead of a 12ga, so the Baikal is not the only one to have ever done so.
