Probably not much help Steve, but true Stub Twist barrels were uncommon after about 1860.
The Irish Industrial Exhibition of 1853: A Detailed Catalogue of Its Contents, John Sproule, 1854
http://books.google.com/books?id=cNKl8YYZejsCIn stub-twist, a bar of iron is made as follows: — A quantity of stubs, i. e., small pieces of iron or steel, are raised to a welding heat, and consolidated by a few blows, and then drawn down between rollers to the required size. The excellence of the material depends on the quality of the stubs employed; that being in most repute formerly which was manufacture I from horse-nail stubs, or old horse-shoe nails collected by the farriers. Of late years these have deteriorated in quality, and it became necessary to apply, in the preparation of the strands, other processes which have for their object the purification of the iron...
The New American Cyclopaedia: A Popular Dictionary of General Knowledge George Ripley, Charles Anderson Dana, 1859
http://books.google.com/books?id=kL9PAAAAMAAJFrom a very early period the superiority of the metal used for horse-shoe nails was well understood, and the old nails, or stubs, were reserved for the manufacture of gun barrels. They were largely imported into England from the continent; and it was not until the supply was becoming insufficient that such success was attained in the use of other equally good materials...
The Modern Sportsman's Gun and Rifle, John Henry Walsh (Editor of The Field), 1882
http://books.google.com/books?id=OLwUAAAAYAAJ“CONSTRUCTION OF THE GUN”
The chief difficulty in the present day is to obtain iron of sufficiently good quality to mix with the steel, whether for Damascus or laminated barrels. Formerly horseshoe stub nails were alone thought good enough; but of late years these have fallen off in quality, and are also insufficient for the supply of the increased demand for shot guns since the passing of the present game law.
Shooting Thomas de Grey Walsingham, Ralph Payne-Gallwey, Gerald Lascelles, Archibald John Stuart-Wortley, Simon Fraser Lovat, Charles Lennox Kerr, 1886
http://books.google.com/books?id=MT9NF4BnAFICGun-barrels of the best kind, since the early years of the present century, have been formed by wrapping round a metal rod a bar consisting of hot iron and steel strips twisted up together, horseshoe nails, as before, having been used for the iron till about 1840, at which date the nails went out of use, the supply of these not being sufficient to meet the rapidly increasing demand for sporting guns.
Of course even the U.S. makers referred to barrels as "Stub Twist" into the 1880s.
Parker hammer lifter
If the barrels look like this, the gun is likely of superior quality and dates pre-1860