We see a lot of crossover from period furniture to rifle carving in the c scrolls and floral forms found on both. Yet the scrimmed decorations on powder horns seldom bring to mind anything we see on rifles made at the same time in the same places. Why is that? Consider that a man with a powder horn might very well have a rifle with carving or inlays. Now he sits down to scratch on his horn. What will he put on it besides text? Seldom will we see baroque or rococo designs executed in ways that occur on contemporaneous guns, or other symbols which may have once or at that time had deeper meanings.
Just felt inclined to offer my twopenn'orth
I think you are simply looking at differences between the products of trained craftsmen and the result of everyday folk trying to personalise their possessions, record their experiences, or simply pass the time away when kept home by bad weather.
The transfer of motifs from architecture and furniture to gun making seems quite predictable. Acanthus leaves, C scrolls etc. would have reached America quite early on, as part of the repertoire of carpenters, cabinet makers and even gun makers as well as on imported furniture. They would soon be seen on better quality items of local manufacture. Decoration doesn't make a gun work better, but as a gun maker, if you thought that some carving would make your guns appear to be a better quality you wouldn't hesitate. You would use the latest style and would certainly be able to provide yourself with any additional tools required.
I would hazard a guess that for most people, carvings on guns would be the only place they would encounter these designs. Few would be gazing across the street at ornate buildings or be invited in to admire chairs from the old country. Rococo simply would not be a big part of the visual world.
Think now about an ordinary chap with a nice new powder horn and some time on his hands. I suppose the first impulse of most people would be to inscribe their name. If that goes well, what next? Not crosses - the widespread use of religious symbols had pretty much disappeared with the medieval era. Certainly in 18th century England at least, too many crosses about your house or person could be seen as a bit suspicious - there were still a number of privileges denied to Catholics. Not Acanthus leaves - not a familiar sight, rather hard to draw (as I've been discovering) and I don't think they work well as a simple line drawing anyhow.
I haven't seen many powder horns, but I have seen collections of scrimshaw done by sailors, also unsophisticated men with time to fill. They seem to go for scenes from naval life - ships, diagrams of sea battles, little maps, names, dates and simple decorative bands. Things they had probably seen for themselves. I'd expect a man with a powder horn to do likewise. Some would discover a talent and go on to produce more elaborate work. Some would simply have more to say. Either way I would expect their work to be quite distinct from professional work. The inspiration is different and the idea of having everything match is still in the future.