This is a .22LR cartridge designed to the real cartridge specifications for the round, although I did make some slight changes (IIRC) so that it can be printed on a FDM/FFF printer at 0.2mm layer height and actually be grabbed by an ejector (and so used as a snap-cap if desired; see below).

If you do want to use this as a snap-cap, print it at 0.15 to 0.2mm layers using a flexible filament with several walls/top/bottom layers and near-solid infill. Make sure to print it slower than usual and with higher-than-normal temperature (and no nozzle fan, preferably) so that all the layers fuse together really well; if not the rim will probably get ripped off when you try and eject the round.