I believe that [b]properly made and fitted[/b] plate mail is rather mobile, but mass produced/secondhand make wouldn't fit proper and would limit mobility understandably- but even then, not too horribly negatively. 

Plate mail is also, for the most part, capable of sustaining black powder rifle shots-- to the torso, at least. Not many, but one shot wouldn't penetrate fully. It'd crush the armor in and likely kill them anyway, but the armor could be fixed up good as new rather easily-

But this is neither here nor there xD