Mr. No gold--Dick is right.
It is undoubtedly a barrel made by William Pannabecker - the son. William senior made complete rifles and barrels, but his son William (active 1835-1878) was one of the many barrel makers along the Wyomissing Creek near Mohnton, Pa 7 miles below Reading. We see William Senior barrels on many Wolfgang Haga rifles---Kindig has about half of his Hagas with Pannabecker rifles.
William Junior was a prolific barrel maker, even managed a barrel factory in Trenton, NJ during the later part of civil war. Returned to Mohnton and rented the house next to my GG Grandfather another barrel maker trained by Henry Deeds, a major barrel maker on the same creek. From 1750 on through until the 20th century there were almost 30 barrel makers on a three mile stretch of the Wyomissing Creek, a few of their dams still exist and one mill still stands.
I assume your barrel mark is on the lower flat, but I've seen Pannabecker's name (a stamp) on the side flats as well on John Derr and Upper Susquehanna rifles. The other Pannabeckers, Jess and Samuel made rifles almost always seen with script signatures on the top flat. A few other brothers made barrels and rifles but we rarely see their products..
Sorry, for offering more than you probably cared to hear...
Patrick Hornberger
EASTWIND