Predictions....

[Furious Mental]

Well I don't think a shield would be much use against guns but still it would have offered the same protection against melee weapons, and against bows (which were still in use in various places). But really if you were a European cavalryman and you were waylayed by, say, a Mughal cavalryman wearing brigandine or plate and mail, the obvious thing to do would have been to shoot him with a pistol. Problem solvered.