Many of the octagonal barrels did NOT have the bottom three flats filed and blued/browned. And the channel left round-bottomed. As Bill mentions above, DO have the very front and rear octagonal to match the barrel.
Gunline, who make checkering tools, also has both round and octagonal barrel channel scrapers in various sizes. Their tools are available through Brownell's, or just look online for them.
If you are not familiar with scrapers, they are flat pieces of tool steel that have been filed on the edge to produce a very small "edge". A lot of builders use scrapers instead of sanding. They are capable of scraping off a lot of wood pretty quickly, if you want them to, or just a minute sliver if you want that. usually leaves a surface that is ready for finishing, and a wood/metal fit that is really tight.
