Interested in loading black powder shot cartridges for a Maynard Second Model percussion style.  The chamber measure 0.694" diameter, so that makes it a 14 Gauge (0.693") according to my book.

Rocky Mountain Cartridge lists a .64 gauge brass case, but for a 1883 Maynard, so that case has a primer pocket in the base, whereas the 1865 would have a flat base with a "flash hole" to pick up the spark coming from the percussion nipple.

I have some Magtech 12 gauge brass cases, however at a diameter of 0.729" they would not chamber.

Anyone use modern plastic high brass cases in the Maynard?  Suggests and/or your experience would be appreciated to get me out in the field this hunting season.

C.M.M.