Here is a 3d-printable compression nut with 7/16-24 threads. My printable cannons all use female threads for the breech, but the breech is oriented 90 degrees to the rest of the cannon and thus FDM printers have difficulty printing this portion. The solution is to print a separate compression nut, then solvent-weld it to the rest of the cannon.
I have included the original Fusion 360 file so you can edit it yourself, or you can use any of the various .stl files. The .stls have been scaled up by the listed amount, so you may want to try a few to find out what gives best results with your printer.