This design is inspired by fléchette spectre.
I design this as I'm using a 0.6 nozzle diameter, this model is able to print with a 0.4-0.6 nozzle as the walls are thicker. Maybe slightly stronger?
Breech is meant to fit in 19/32 brass size. This set up will be much easier to build a brass breech. Unlike back then everyone has to cut the nub and glue on the brass which is a hassle.
To print this it will be better to use cura slicer, as the 100% will covered everywhere. I'm not sure about other slicer but cura is the only one I used.