This is a 5.56x45mm NATO cartridge designed to the real cartridge specifications for the round...although I think I may have made thickened the rim a little bit so that it could be chambered and ejected without the rim simply being ripped off (but I made this awhile ago so I cannot recall for sure. Sorry). The bullet-tip has been rounded off as a pointed tip likes to print like a soft-serve ice cream cone; even so, I suggest printing the topmost layers slow and with the nozzle fan running at max speed (if you're not doing so for the whole thing).

If you 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. As an alternative you can print in a rigid filament, using multiple shells/walls (Probably 5 or more) and NO/0% infill and NO bottom layer(s) - that way it will leave open an "open tube" down the middle of the bullet for the firing pin to go into without hitting the plastic.