Dan - Yes - turned for a guide starter for sure. For others here who are interested but without any experience in this, if it doesn't have locater pinholes for a false muzzle, then guide starter and plunger looks to be THE tool for this barrel. The pins of a false muzzle of course, would create a more accurate fit than something sliding down the side of the barrel.
Good suggestion on 'filling' a round, snug fitting guide with epoxy or pewter - I suppose most metal bearing epoxies would work, like Devcon or J*B Weld. Dang - I shouldn't be getting wound up over shooting bullets in my .40. Making a guide starter of bronze or hard brass would work on an octagonal barrel as long as the points of the flats are a press fit on the bore of the starter body.
The .018" depth of rifling is a puzzle, when only .008" to .010" would work find. This 'extra' depth suggests a fairly small bullet, maybe .005 to .010" under bore size and heavier patch. The guide with wide rounded dome 'striker' would be necessary for starting a bore-sized picket with heavy patch- but then, I load and shoot balls .002" larger than my .40's bore with .0215" denim patches- measurement comes out to .443 in a .422" groove depth. This loads fairly easy once started. One smack with the flat end of my starter puts it level with the crown. A .40 with .018" depth rifling would be easier due to larger groove diameter - in a true .400" bore and that rifling, the groove depth is .436", still .007" compression in the grooves or .0035 per side. Have you tried this sort of combo with pickets your Reinhard, Dan? The patching is 10 pound Denim @ .0215" thickness on my mic., measured dry. I always use the mic, now as it gives consistant results - my calipers don't.