Louis: do you have a copy of "The Longrifles of Western PA" by Rosenberger and Kaufman? There are a couple of T. Allison rifles featured therein in black and white, and in colour...but unfortunately, no close up pics of the entry pipe. From what I can see, the barrel of the pipe is ~ 1 1/2" long and the skirt is a little longer than that. the barrel appears to have raised bead mouldings at each end with flats filed between.