Ohio rifles are dear to my heart. I have a ton of (collected) research on them.
What confuses the matter with "J. McCammant" (spelled various ways) is that there were two of them that signed their rifles that way.
James McCammant, "the father", worked as a gunsmith in Washington Co. Pa, Brook Co. (West) Va, and Wellsburg, Va before moving on to Ohio (Perry Co).
His son John McCammant worked as his partner for many years. They ran a brass foundry in Virginia and built rifles. John remained in Virginia after his father relocated to Ohio, but ultimately (sometime after 1854) moved to Illinois (Montgomery Co) where he "built and repaired - rifles, fowling pieces and holster pistols".
I do have 7 or 8 more pictures of this rifle, mostly just close-ups of the other inlays (unfortunately, no close up of the breech end - and no other measurements) but do have a note that this had "light octagon barrels" (
)
** I did make an error in my original post, stating that he ended up in Ohio - confused with his father **
(if my history is correct on this family of gun makers, this rifle would have been built in Virginia (Wellsburg) very close to the "National Road")