I have one of those GM tapered octagonal barrels in a Winchester heavy #3 or #3 1/2 taper - 35" long, 1" at the muzzle. Mine is chambered for the 1.9" case as I used my .458 2" reamer "held back". I had it threaded for my modified 1868 Sharps action.
With it's fairly long leade, smokeless loaded bullets can be seated out considerably and with it's bl. length, it gets approximately 250fps higher velocity than what is noted in Hodgdon's data - ie: for their listed 400gr. jacketed at 1,640fps, I get 1,850fps a pressures well under 24,000CUP. My loaded length is the same with those bullets as if loaded for a Marlin. Of course, my rifle cannot take the pressure the lever guns do, so I hold them to loads listed below 28,000CUP.
1,850fps is not shabby for a Sharps with 400gr. bullets.

Incidently, this barrel and load made a witnessed 7/8", 10 shot group at 100meters, off bags. Very accurate barrels. Got mine from trackofthewolf. This was when using a cheap $300.00 tang mounted aperture sight and #110.00 hooded front globe (Sharps design) with aperture.

With an original Sharps Carbine open rear sight and silver blade front, I've made a number of 1 1/2" 3 shot groups at 100 meters, witnessed by my brother, using 42.0gr. Benchmark and 506gr. cast RN w/GC @ 1,500fps.

Note the action is a modern Italian action in 4140 steel. The design is from 1868 - so low pressure only.