Maynard did in fact offer an interchangeable shotgun barrel in 20 ga. They are mentioned in the later catalogs if memory serves. Over ten years ago I found the one I now have at the Baltimore show peeking out from under a pile of WWII web gear as I was on my way to the exit to head home. It has 100% original blue and truly a mirror bore. Further, it is complete with cased lever, a link with an unmolested extractor pawl and link screws which have not been turned out since they were set by the factory.
This condition has allowed me to make what I feel is an interesting Maynard observation about, at least , this barrel. It has a cylinder bore, however the muzzle is ever so slightly uniformly constricted, or choked, for a fraction of an inch by seemingly having been pressed or driven into a cone shaped die. There is no evidence that this was haphazardly or accidentally done. You can detect it on the outside by feel with your fingers and internally with a firm patch on a cleaning jag.
A second notable point is that 20 ga. shot shells of the period can be reprimed using the same plier tool made by or for Maynard to reprime their unique large base 1873 model bullet and shot cartridges. If your shot shells are of the Berdan type, the same is true that a Maynard cap picker will serve. This barrels is generally interchangeable with any 1882 model Maynard I have, but usually it is paired with a very late 1882 original .32-20 in nearly the same condition. Someday I hope to make a period style walnut case outfitted with all original loading tools for this two barrel small game set
