Pepperboxes were common and popular in both Europe and the U.S. even after the invention of the revolver. Some had barrels drilled in one piece of steel/iron; others had separate barrels bundled together with ribs so as to appear to be one piece; others had separate barrels clearly separated.
They were usually made in what we would call double action only; single action ones and SA/DA ones are known but rare.
While pepperboxes undoubtedly had a high "fear factor", they were heavy and unwieldly. and the DA pull was horrible. Almost needless to say, if any two barrels shot to the same point of aim, it was pretty much coincidence.
More information will have to wait for more details and/or pictures.