The advent of smokeless powder didn't cause their demise. These George H. Fox designed sideswing hammer doubles were gone before the advent of factory loaded smokeless powder cartridges in the early 1890s. American Arms Co. by then was making cheap revolvers, side-cocker single barrels and a semi-hammerless side by side, and introduced their high end Whitmore-designed hammerless double --
The best articles on American Arms Co. are one by Elliot L. Minor in the April 1970 issue of
The American Rifleman, and the one by William M. Furnish and James A. Nelson in
The Double Gun Journal, Winter 1994.